In all the material that I've covered from the Star Wars Universe, apart from Vader, I have not read of any one who came to the Light Side of the Force after being exposed to the Dark Side. Given that my knowledge is limited, I would like to ask whether there has been any one who came to the Light Side of the Force after experiencing the Dark Side?
Well, one example would be Darth Caedus, aka Jacen Solo; he went back and forth a few times. It's not an uncommon theme for someone to be tempted (or pushed, as in Jacen's case), go over to the Dark for a while, and come back. In Jacen's case, he swung back again, and ended up
dying while on the Dark side
but a number of characters have spent time on both sides.
If you can add more detail to the question, I might be able to provide a more accurate; Luke spent time yielding to the Dark Side in the original trilogy, but didn't actually go over. Does he count? Are you looking only for those who self-identified as Dark (i.e., Dark Jedi, The Sith, Bando Gora, etc.), or are you looking at those who wavered, but stayed light? And are you looking for ones that had a Dark origin, or only ones that went that way at some point? Also, what degree of Canon?
There are some pretty funky examples of each; the Star Wars game, for example, reference the 'Light Sith' who are trained in the Dark Side, but reject it for the light. (The Dark Jedi is a bad term, as it contains several groups, but it originally referred to the Jedi equivalent; those who fell from the Light.)
Since osdamv unfortunately declined to post "Revan" as an answer, I will.
Revan, renowned as "The Revanchist" and dreaded as the Sith Lord Darth Revan, was an eminent Jedi Knight turned conquering Dark Lord of the Sith until, stripped of his true persona, he returned to the crumbling Jedi Order and helped defeat the Sith Empire he had established
Revan was a fairly major figure in SW canon. Among other things, he:
Defeated none other than Mandalore the Ultimate and won Mandalorian Wars while a Jedi
Took over Rakatan Star Forge while a Sith
Started a Jedi Civil War while a Sith Dark Lord
Driven Sith off of Korriban after becoming Jedi again (though involuntary, since he lost his memories)
Was instrumental in helping Jedi win Jedi Civil War, after his full memories were returned and he chose the Light side consciously.
Later, was presumed dead while fighting Sith Empire.
Rajivari (post mortem), Ajunta Pall (post Mortem), Kel'eth Ur, Gav Daragon, Ulic Qel-Droma, Revan(though he seems to have fallen again), Bastila, Malak (at moment of death), Sion, Atris, Possibly the SI and SW from TOR if LS is determined canon.
Skipping to more recent times...
Darth Gravid, Anakin Skywalker, Luke Skywalker, Kam Solusar, Kyle Katarn, Kyp Durron, Zekk, Jaina Solo, Ben Skywalker, Tahiri Veila (I'm not counting Caedus. I don't believe he was redeemed.)
In the Dark Empire storyline, Luke faces the Emperor reborn and decides he must face the Dark Side "from within." (Issue #2) He starts down that path and is brought back with Leia's help.