I use a combination of the Rokugan taint rules, the Conan RPG stuff from Mongoose, and also the section on the Delifers and Preservers that was written up in Dragon for the "Dark Sun" section of the Campaign Classics issue.
It works pretty well. In order to not completely nerf my characters, I basically only apply it to arcane magic from wizards and sorcerers (not bards - that magic comes from a different place IMC), and it does not apply to 1st and 2nd level spells. Only when they start using 3rd level spells do they become aware that the magic can corrupt them. I make them make Will saves at DC 10 + Level of the Spell they're casting. On a failure, they get a "corruption" point. After they've gained 3 Corruption Points, they gain a free metamagic feat that they qualify for (without raising the level of the spell, but it only applies to certain spells and I don't tell them that it's happening), but when they use that feat their spells gain something kind of like the "evil" descriptor. The character also has to make a Wisdom check to actually know what is happening. If they fail their Wisdom check, I let the player know, via notes or an e-mail or something that "You feel powerful! Your spell seemed to last longer/cover a wider area/etc. You feel the urge to cast more spells to try it out." If they make their save, I let them know that they feel a little strange, as though some kind of outside force is trying to change them and make them do things differently.
Basically, I give the player ample opportunities to try to recover and get rid of the Corruption Points. If they don't, though, over time they're lose a few points of Wisdom, become Evil, and cast their spells with the Evil descriptor. The severity depends on whether they willingly let things happen, or just get sucked into it unawares.