Adding another link: PaperBackSwap.com
This is an online book club where you can swap books with other members for the price of postage. It's great for people like me who a) have about five zillion books languishing around that used bookstores won't take and b) have a list as long as their own arm of older books they just can't find anywhere. Fix both problems at once.
Nicely browsable and searchable, with publishers' blurbs and reviews.