Yes. The article on the Dark Side of the Force on wookiepedia lists several organizations that exist in canon as users of the dark side:
Dark Jedi: Probably the most confusing group, Dark Jedi either were former Jedi who chose to follow the dark side or those with no light side training that simply chose the dark side. The difference between them and the Sith were the specific teachings of the Sith, however they occasionally worked along with the Sith.
Nightsisters, or Witches of Dathromir: The best-known group of dark side users outside the Sith themselves. The dark side users of a shamanistic culture, they led the matriarchal tribes of Dathromir. Noted for riding rancors.
Rakatan Infinite Empire: The first known galactic government, they enslaved billions and used dark side powers to fuel their war machines and to create more with the Star Forge.
Prophets of the Dark Side: Technically a splinter faction of the Sith, this cult was founded by a renegade Sith Lord and eventually rose to some prominence under Emperor Palpatine.
Bando Gora: A dark-side worshiping criminal organization, singled out as a threat by both the Republic and the Sith.
Sorcerers of Tund: Reclusive group of dark side mages native to Tund. Rarely ventured off-planet.
Edit: In the Legends novel Darth Bane: Dynasty of Evil, I found the following passage (p.68 of the hardcover edition). This provides an Old Republic Jedi's perspective on the difference.
"What's the difference between a Sith and a Dark Jedi?" Lucia asked.
The Ithorian stopped pacing and turned to face them, instinctively addressing his audience like a teacher giving a lesson.
"The Sith were the sworn enemies of the Jedi and the Republic. They sought to wipe us fro existence; they sought to wipe us from existence; they sought to rule the galaxy. They united their strength in the Brotherhood of Darkness, drawing countless followers to their cause with false promises. They amassed an army of individuals foolish and desperate enough to believe their lies, and they plunge the galaxy into a war that threatened to destroy us all."
...
"A Dark Jedi, on the other hand, has much smaller ambitions. He - or she - thinks only of himself. He acts alone. The ultimate goal is not galactic conquest, but personal wealth and importance. Like a common thug or criminal, he revels in cruelty and selfishness. He preys upon the weak and vulnerable, spreading misery and suffering wherever he goes."