Hmm. Not sure. I know Sith cross back over and redeem themselves... hell, happens kinda frequently.

ok need to read more !!!

Dam work!!!!

I suppose you can count Vader becoming Anakin again.

Hiram, not sure on that.
Anakin was a Jedi, then became a sith by necessity - to save Padme, not just because he lusted power.
After 20 years of terror for the galaxy - and continued slavery for Anakin - he partially redeemed himself by carrying out another act of necessity, saving his son.
I'm not entirely sure if that really counts.
Even Dooku, who was one of the lost 20, dallied with the idea of turning back to the Jedi. Maybe because he forsaw the path Palpatine was taking re: recruiting Anakin to his side.

'Tis a strange one this. Even the mighty Sith Empire of 4 millenia or so before the movies were descendants of fallen Jedi who mastered the indigenous races that they fled to (on Ziost, Korriban etc), so whilst they were born Sith, as opposed to being Jedi who became Sith, I suppose it is entirely possible for the Sith of that era to fall to the Light Side.

However, there has never been any fiction to support this theory. I suppose that if any Sith of that time had strange notions of being good and kind, they would be brutally murdered and tortured, and hushed up, and not necessarily in that order.

Well, from the films we see mostly the fall from Jedi to Sith, with only one case of this transition being reversed. I think this stems from the simple fact that Jedi limit their use of the force while the Sith do not and, human nature being what it is (they are mostly humans, after all), it is much more likely that one who has power will give in to the temptation to use it than that one who has and uses power will voluntarily turn away from it. Vader's redemption came only in a moment of terrible realisation when faced with the choice of serving his Emperor or saving his son.

In short: falling down is easy, climbing up is not.

suppose that if any Sith of that time had strange notions of being good and kind, they would be brutally murdered and tortured, and hushed up, and not necessarily in that order.

This kind of reminds me of how Mean Machine Angel was created. Remember that?