One of my favorite places on the web is Librarything. It is a book cataloging site, which allows you to enter 200 books free and see who shares books with you and a plethora of other services.
Heres a link to a the site "About US": http://www.librarything.com/about
Here's my library: http://www.librarything.com/catalog/SankofaAckee
My favorite library on the site:
Another site I enjoy quite a bit is bookmooch, a book trading site. All you have to do is add enough books to get 1 point and you can order a book from someone else's collection for free. I have gotten books that costs up to $150.00 retail off the site and was clear my of the book I found unworthy out of my collection. Best of all its connected to librarything.
So check it out here: http://www.bookmooch.com