There are a number of elements from the EU that were borrowed by George Lucas and incorporated into what was then known as G- or T-Canon; most notable among these are Boba Fett (who, as noted by Valorum in a comment on the question, originated in the Star Wars Holiday Special) and the planet Coruscant, which was created by Timothy Zahn for the 1991 novel Heir to the Empire (though based on concepts from earlier drafts of the films).
To be perfectly clear, this list is only EU elements that were referenced in canon materials published after the announcement of the Legends banner in April 2014; this quite deliberately excludes The Clone Wars: The Lost Missions (the previously-unaired season 6 of The Clone Wars animated show).
Two versions of the Jedi Code:
The longer version, which in the EU was formalized by Master Odan-Urr and remained in use basically forever, was used in the canon novel Dark Disciple:
There is no emotion, there is peace.
There is no ignorance, there is knowledge.
There is no passion, there is serenity.
There is no chaos, there is harmony.
There is no death, there is the Force.
Dark Disciple Chapter 34
The shorter version, which appears to have been originally introduced in one of the roleplaying sourcebooks, was canonized in an issue of the comic series Kanan:
Emotion, yet peace.
Ignorance, yet knowledge.
Passion, yet serenity.
Chaos, yet harmony.
Death, yet the Force.
Star Wars: Kanan 007
The planet Nar Shaddaa, a crime-ridden ecumenopolis, was a main setting of the second story arc in the new Marvel Star Wars comic book series1:
The Smuggler's Moon was also a mainstay of the EU, dating all the way back to Dark Empire and appearing in many, many works:
The Imperial Interdictor was first seen in the Rebels episode "Stealth Strike" (though they were mentioned in the new canon novel Tarkin):
It bears a striking similarity to the EU's Interdictor-class Star Destroyer, first seen in one of the Rogue Squadron comics, but is more likely based on the Sienar Immobilizer 418 Interdictor cruisers from the West End Games Imperial Sourcebook2:
Tarkin specifically mentions and describes an "Immobilizer 418":
In the distance, positioned against a radiant sweep of stars, floated three Interdictor vessels, a Detainer CC-2200, a newermodel CC-7700 frigate, and - fresh from deepdock in the Corellia system and as yet untested - an Immobilizer 418. Thickly armored, the former two had downsloping bows and stubby winglike lateral projections housing quartets of gravity well projectors. The Immobilizer, by contrast, featured four hemispherical projectors aft on the ship's sharp-bowed hull.
Tarkin Chapter 17: "Zero Defects"
The Outland Regions Security Force, a militia based on the planet Eriadu and tasked with defending the Greater Seswenna region from marauders and pirates, was introduced to the EU in the 2000 novel Rogue Planet. In the novel they were said to have been created by Ranulph Tarkin, elder cousin to the more famous Grand Moff Wilhuff Tarkin, who would later join the group.
Much of this was retained in the new canon novel Tarkin: The Outlands Security Force still exists, and for the same purpose, but Ranulph Tarkin is now a well-respected past commander rather than its founder. Wilhuff Tarkin (still a cousin of Ranulph) joined the group as a young man, rising to the rank of Commander before enrolling in the Republic Judicial Academy.
The institution of the Imperial Inquisitors, a major part of the first two seasons of Rebels, was also originally an EU element. Such luminaries as Jerec, primary antagonist of the 1997 video game Jedi Knight: Dark Forces II and Brakiss, infiltrator of Luke's Jedi Praxeum, were EU inquisitors.
This is somewhat debatable, but the Hammerhead corvette made its first canon appearance in the Rebels season 2 episode "A Princess on Lothal":
These ships are extremely similar in design to the Hammerhead-class cruiser introduced in the Knights of the Old Republic video game:
Grand Admiral Mitth'raw'nuruodo, Thrawn for short, was the primary antagonist in season 3 of Star Wars Rebels, and may continue into season 4. Thrawn was first introduced in Timothy Zahn's Heir to the Empire novels, and he remains one of the most popular characters of the old EU.
Many aspects of Thrawn's backstory (including his full name) were re-canonized in the 2017 canon novel Thrawn, written by Timothy Zahn; examples include:
- The Chiss Ascendancy and the Aristocra, respectively the government and ruling class of Thrawn's homeworld
- The Sy Bisti trade language, spoken by human settlers near the Unknown Regions and known to the Chiss
- Thrawn's exile from the Chiss as punishment for breaking their rule against pre-emptive strikes
- Many details of Thrawn's discovery by Imperial forces; aside from the omission of Booster Terrick, the first chapter of Thrawn is a point-for-point retread of Timothy Zahn's 1995 short story "Mist Encounter", which tells the same story
Thrawn also brought back the material doonium, which is used in the construction of starships. In the EU, it originated with the Jedi Prince series
Rukh, Thrawn's bodyguard from the Legends novels, was confirmed for season 4 of Rebels at Celebration Orlando 2017
How much similarity he will bear to his Legends counterpart remains to be seen
The phrases "Ashla" and "Bogan" to refer to the Light and Dark sides of the Force were used by Bendu in the season 3 premiere of Rebels:
Bendu: Jedi and Sith wield the Ashla and Bogan, the light and the dark. I'm the one in the middle, the Bendu.
Star Wars Rebels Season 3 Episode 1: "Steps Into Shadow"
The word "Ashla" had previously been used on Rebels, and was obliquely established as the name given to the Force by the Lasat people:
Chava: The Ashla led us to the pirate, then to you, so that we may find our path to the new world.
Ezra: The Ashla?
Chava: The spirit of the galaxy.
Ezra: Sounds like the Force.
Kanan: The Force has many names, Ezra.
Star Wars Rebels Season 2 Episode 12: "Legends of the Lasat"
These terms were first introduced in the Dawn of the Jedi comic book series; they were the names of the twin moons of Tython, the home planet of the proto-Jedi, and were often used metaphorically to refer to the two sides of the Force.
The Imperial Security Bureau was first mentioned in the Rebels prequel novel Ezra's Gamble, released in 2015. ISB Agent Kallus has been a recurring character on Rebels since season 1.
The ISB was also frequently mentioned in EU materials; can't have an evil, oppressive empire without a secret police.
Admiral Yularen's career as an ISB agent was a relatively unremarkable part of the EU, but it was carried into canon by the novel Tarkin:
Close to Tarkin's age - though with more gray in his hair and especially in
his bushy mustache - Yularen had traded a distinguished career in the Republic Navy for a position in Imperial Security, heading a division devoted to exposing instances of sedition in the Senate.
Tarkin Chapter 8: "The Emperor's New Spies"
Although Yularen appeared in A New Hope (he's the white-jacketed gentleman in the Death Star meeting room), and was still an ISB agent at that time, I feel justified in including him here; he was an unnamed background character in that film, and his backstory was retconned in The Clone Wars show and later novels (like Tarkin)
As I mention in my answer here, the Nabooian king Veruna, Padmé's predecessor first mentioned in the Legends novel Star Wars Episode I Journal: Queen Amidala, has been ported to canon as of some DK reference books published in 2016
Two versions of "spice", the Star Wars universe's favourite recreational drug; Spice itself has a long history in the EU, but these specific varieties are new to canon:
Ryll, a mineral native to the planet Ryloth which can be refined into a drug, was introduced to Canon in the novel Lords of the Sith; it's a major factor in the Empire's occupation of Ryloth, which is the inciting factor for much of the conflict in the novel.
Glitterstim was canonized in the canon novels Battlefront: Twilight Company and Bloodline
Another somewhat debatable one, but the knobby white spiders3 were creatures native to the planet Dagobah, introduced in the EU novel Darksaber:
A very similar-looking creature, called the Krykna, a native of the planet Atollon where the rebels were trying to establish a base, was introduced in the Star Wars Rebels episode "The Mystery of Chopper Base":
The resemblance isn't unusual, since both creatures were based on (unused) concept art by Ralph McQuarrie for Return of the Jedi.
A couple of corporations:
- The Slayn & Korpil corporation, manufacturers of the Rebel Alliance's B-Wing fighters. In Legends, they were introduced in one of the West End RPG sourcebooks; in Canon, they're mentioned in the Ultimate Star Wars DK reference book
- Czerka Arms, the weapons manufacturer introduced to Legends in one of the West End RPG sourcebooks and a frequent background antagonist in the Knights of the Old Republic video game series. They were canonized in the Ultimate Star Wars DK reference book (where it's revealed that Aurra Sing uses a Czerka rifle), and in the canon novel Aftermath
Some of Chewbacca's family:
His wife Malla is first mentioned (in canon) in the new young reader's novelization of A New Hope, after Han strikes out:
"Laugh it up, buddy." Seeing Chewie's look, Han added, "Hey, you're the married one. Find me a human girl with Malla's brains and silky hair and maybe I'll consider it."
The Princess, the Scoundrel, and the Farm Boy Chapter 7
His son Lumpawaroo (diminutizes as "Waroo") was canonized in the 2017 novel Aftermath: Empire's End:
The bandolier Wookiee sees Waroo and cocks his head. He utters a soft purr before the strength seems to go out of his legs. Waroo knows him. This is his father. This is Chewbacca.
Empire's End Interlude: "Kashyyyk"
Both Mallatobuck (her full name hasn't made the canon transition yet) and Lumpawaroo (though not named until later works) were created for the infamous Star Wars Holiday Special, but would go on to appear in other, less silly EU novels and comic books.
Details on the Battle of Hypori, which appeared in the Legends animated series Star Wars: Clone Wars (not to be confused with the fully-canon, CG-animated Star Wars: The Clone Wars). This battle is referenced in the canon novel Catalyst, and mentions two facts from the series that had previously gone unconfirmed in canon (emphasis mine):
The hulk of a crashed Acclamator-class cruiser stood as a grim memorial to a pitched battle in which a group of Jedi Knights had been taken by surprise.
Catalyst: A Rogue One Novel Chapter 24: "Cardinality"
Although the death of the Jedi K'Kruhk on Hypori (at the hands of General Grievous, no less) was already canon4, having been mentioned in the Revenge of the Sith novelization, no other details from that battle (nor, indeed, that it even was a battle) had previously been canonized
I've seen it mentioned by several fan sources that (at least some of) the spoken Mandalorian heard in "Legacy of Mandalore" (from season 3 of Rebels) is taken from the EU Mando'a language, extensively developed by Karen Traviss. Some canon Mandao'a seen in prior canon works (such as Beskar'gam, Mand'alor, and Mando'a itself) are also borrowed from Traviss' work
The expletive "kriff"/"kriffing" was initially created by Timothy Zahn in his 1998 novel Vision of the Future, and it soon became a favourite curse in Legends material. It was canonized in the junior novel Servants of the Empire: Edge of the Galaxy, released in October 2014.
11-4D, Palpatine's droid assistant, was a minor character in James Luceno's Legends novel Darth Plagueis, where he initially served the titular Muun, before being claimed by Palpatine after his master's...unfortunate accident.
The droid was referenced again in Luceno's canon novel Tarkin, and retains a similar backstory:
The droid resembled a protocol model, except for its several arms, only two of which terminated in what might be considered hands; the rest were devoted to tools of varied purpose, including computer interface and power charge extensions. The droid had once been the property of Sidious's tutor, Plagueis, and had been in Sidious's possession since his former master's death, though in several different guises.
Tarkin Chapter 9: "As Above, So Below"
As noted by Valorum in an answer elsewhere on the site, Luceno also borrowed the substance meleenium, originally from one of the West End RPG sourcebooks, in the canon novel Catalyst
Kath hounds, which originated as low-level enemies in the Knights of the Old Republic video game series, were name-dropped in the 2017 novel Phasma; since they're never described, it's not clear how closely they resemble their Legends counterparts
"Slicing", the Star Wars equivalent to hacking, was first mentioned in the new canon in the novel A New Dawn. Although hacking technically dates back to A New Hope, with R2-D2 infiltrating the Death Star's computer systems, the notion of human Hollywood Hackers began in the EU, with Timothy Zahn's Heir to the Empire
In a truly fascinating example, Solo (that is, the Han Solo anthology movie) director Ron Howard and co-writer Jon Kasdan confirmed on Twitter in October 2017 that Tag Greenley and Bink Otauna will appear in the film. What relation they will bear to their Legends counterparts remains to be seen.
Tag and Bink were comedy characters who managed to be accidentally present at almost every major event of the original six films. Their exploits were considered non-canon at the time, but were canonized in Legends as in-universe tall tales. Tag and Bink appear to be the only elements of the Infinities continuity to be elevated to Disney canon.
The card game pazaak, which was created as a minigame for the Knights of the Old Republic video game series, has been namedropped a few times in the Aftermath novels. The rules of the game (basically blackjack, but the target count is 20 and each player has a side deck of cards that can modify the count) were ported to canon in John Jackson Miller's novella "The Ride", published in the 2017 novel Canto Bight
The big, impressive Force ability used by Luke in the climax of The last Jedi is likely inspired by Legends:
Luke, while sitting on Ach-To, uses the Force to project an image of himself (and Han's lucky dice) on Crait. The projection is visible to everyone present, including C-3PO, speaks to multiple characters, and believably fakes some physical interaction (Leia grabbing his hand, and later holding the projection of the dice)
Something very much like this first appeared in the 1991 comic series Dark Empire, and was described in greater detail in the 2011 in-universe manual The Jedi Path: A Manual for Students of the Force.
I'm including Nar Shaddaa because of its prominence in the EU, but I'm going to refrain from including other planets (like Taris, for example) which are less prominent in the EU and receive only brief appearances or mentions.