Off the top of my head, here are a few sites that let you work off bonuses using SNGs, with the % of the rake they refund you:

Stars: 100%
Bodog: 100%
Pokerroom (7x): ~70%
Paradise: 10%
Absolute: 10%

I'm sure there are others. You might check out Bonus Whores for more details.

One good thing about Bodog's is that it's bottomless (so is Absolute, essentially); you can withdraw and redeposit for an infinite 10% bonus. The games are pretty soft, but the blind structure escalates quickly toward the end. Good luck.