When I get rid of my own fiction or nonfiction books, I prefer to list them on PaperbackSwap. I’ve gotten a lot of books from that site, and passed on a lot, too.
You can create a wishlist of books you want, and when they are available, you’ll get an email and a chance to claim the book. If you want a specific title your library doesn’t have, this is a great way to get a chance to read it. If you REALLY want a book on a time crunch, it might not be the best solution. But if you search a book and a member has it, you CAN get it immediately. The wishlist is just a nice way to keep track of what you want.
If a book won't sell for much on other sites, I'll list it on PaperbackSwap for awhile so someone who really wants it has a chance at getting it.
I like the idea of passing along books when you're done with them, but I admit that I've kept several for my collection, too! I don't mind used books - I actually prefer them because once they get a little worn or bent, you won't feel horrible for ruining a brand new book. I especially like them if they're discarded library books because I love seeing the circulation systems other libraries use.