GoodReads First Reads
I did win a prize pack from HarperCollins Canada on Facebook, but most of my free books come from GoodReads. I just go, click a few buttons and I'm entered to win. The real beauty of it though, is that when I win, it doesn't require any action from me to receive my books. They just show up in the mail, often unexpectedly. Like, I came home today and there it was, sticking out of my mail box - a copy of Bloodman by Robert Pobi. I didn't know it was coming, but there it was. (I'm not very good at email sometimes. I almost missed out on that five book prize from HarperCollins because they needed me to reply to their e-mail with my address. I would have been so angry with myself! Everything worked out for the best though.) If you win, you are not required to write a review, but you can if you're so inclined.
The other thing about GoodReads - even if they didn't keep sending me free books, it's still a great site if you're a lover of books. It's full of reviews and ratings and will even recommend books you might like based on your ratings. You should go there and check it out.