I have a Kindle, too, and looked at other websites but mostly use Amazon! It seems like almost every weekend, they have different books for free, and I've been downloading a lot of those. I've read many authors I've never heard of and probably wouldn't have read if I'd had to pay for the book, so this has expanded my horizons, so to speak.
The other sites I've found with free books are mostly the classics, and since I graduated with a degree in English, I'm pretty much tapped out on classics. Now I read for fun.