I recently came across a great website:
On this site, you can collect all the books you've ever read, any reviews you might have, and new books that you'd like to read. You can rate the books you've read (simple star system) and write reviews of your own. As well, you can add friends and see what they're reading or what they recommend.
There are also a lot of great discussion groups, and I've found quite a few new books to read, from many of the chats that I participate in (books of the month, that sort of deal).
The site also offers up some fun widgets for you to use on your own blog or web page - see mine to the right.
Anyhow, thought this site made a good fit, for my "Web Site Recommendation" blog.
You should check it out!