BookMooch is an excellent book swapping site developed by John Buckman. You can trade your old books to points, which you can use to mooch books from others. It's all very simple, but works really well.
This service is brought to you by Mikko Saari (tip me!). The basic data comes from BookMooch APIs. Thanks to John Buckman for that. LibraryThing's ThingISBN is used to improve the results and finally, Amazon provides the book information.