Goodreads is pretty cool. For the last two years, I've been keeping track of the books I have read (inspired by Kevin Drum) so it's fun to rate them on Goodreads and see what your friends thought of them as well.
However, I think the site could use a lot of work. After signing up, it's very difficult to figure out how to add books to your list. Adding books is easy (you just rate them) but it's not obvious at all that that's how it works.