Who Are Some Villains That Have Become Good?

Not up to speed with current comics continuity, but Taskmaster started off as an Avengers villain and has been used more now as an antihero, though he does still seem to vary. IIRC, Hawkeye was a villain in his first appearance. Still with Marvel, I think Crusher Creel has helped out the heroes on rare occasions, apparently he's been used in Agents of Shield a fair bit, though I don't know if as an outright villain or as an antihero. Maybe Magneto, he did lead the X-Men for a while.

Can't think of many other DC ones apart from the ones alteady mentioned. How about Anarky from Batman, is he a villain as such?

Black Widow and Hawkeye were villains when they first appeared.

Same with Scarlet Witch and Quicksilver.

Not sure if he counts...it's not Eddie Brock (who was never a hero, he was just a psycho who thought he was good) whose Agent Venom now. It's a different guy.

He wasn't much of a hero in the 90s...in the "Separation Anxiety" story when he was defeated by the Scarlet Spider he didn't have the symbiote's influence in his mind and he had no choice but to reflect and admit he was just a killer who was always lying to himself. And as Anti-Venom he'd been more or less altered by Mr Negative into someone else.

Didn't carnage die a hero at one point?

To my understanding.
Banshee- don't know if this counts, since the first pages of his 1st appearance portrayed him as a baddie, and then the resolution revealed he was a coerced good guy.

The Changeling- worked for an evil mutant group (Factor Three). Found out he was terminally ill and asked Xavier for some redemption. With Xavier's blessing, he masqueraded as the Prof himself (while the real one dealt with a time-consuming agenda), and died in a heroic act, making the X-Men believe they lost the real thing.

Rogue- went after Carol Danvers (Ms. Marvel/future Captain Marvel), accidentally permanently stole her powers/memories/emotional essence and (perhaps insanely) tried to dispose of the body. Took down Cap and Thor. Because of the Carol problem (and her lack of control of her absorption powers), went to Xavier for help.

Magneto- after his rejuvenation (one of his mutant creations turned him into a baby, and a Shiar agent restored him to adulthood) wrecked a Russian village (but gave time for mass evacuation) and sunk a submarine, giving its crew a slow, painful death. Due to self-awareness about the justification of his agenda (harming innocents, making things worse for mutantkind), a relationship with a human, and forced alliances to the X-Men, he decided to try another way. He went to Trial, but certain situations (Xavier being forced to take one major leave of absence) led to him replacing Xavier as Headmaster of the School for Gifted Youngsters.

Mystique- tried to assassinate Senator Kelly. Decided on self-preservation and became a federal agent.

Gambit- thief, assembled the Marauders (Claremont's original idea was that he was the 'Hero' façade of the person who also created the 'villain' façade known as Mr. Sinister). Became partners with Storm and followed her to the X-Men.

Emma Frost- White Queen of the Hellfire Club. Kidnapped and fought the X-Men and the New Mutants several times (she also provided therapy to the latter group after a traumatizing experience). Lost her students the Hellions, and decided to try out Xavier's way, becoming a teacher to the next generation of Xavier's mutants (Generation X).

Of course, some of the X-Men heroes had their off-periods, be they brainwashed (Phoenix, Wolverine) or opening dark psyches in their soul (Xavier, Phoenix). And others like Magneto and Mystique flip-flopped (according to the writer or editor).