LibraryThing I love this and paid for the lifetime account. However, you can add up to 200 books for free. The lifetime account was pretty cheap though. I can't remember how much I paid but I believe it was under $20.
Shelfari is prettier. It is also completely free. I use both LibraryThing and Shelfari. I also use Goodreads though not to the same extent as the other two. I love to read and I love to find new books.

: My "to read" list on Shelfari is huge thanks to the recommendations.