You could try selling them on eBay
Or maybe you have a Half Price Books near you? They pay for your used books in cash or credit that can be used toward the purchase of more books right there in the store that very day! (Of course, they aren't going to pay a large amount FOR the books you bring them as it depends on the popularity, condition and genres but still......
I review literary fiction and poetry, so I send the books to friends who teach in M.F.A. programs to have in their department libraries, but some local libraries will take any book in good condition, and I have known some people to sell them on ebay, or amazon.com.
paperbackswap.com - you can post books available and swap for others
offer giveaways on your blog - either individual books, or groupings of books
booklovinbitches(or similar sites) - send some of your favs on "book tours"
offer books or "boxes of books" to your fellow bloggers - lol
I'm always looking for new authors or new books to read, even just for fun - right now I'd LOVE to find good, clean YA, GOOD erotica or romantic suspense, and some good paranormal, especially with shapeshifters