If you like to read (and I really hope you do), you'll like Library Thing. I like keeping track of what I've read, what I want to read, and what I have bought (mostly so I don't check it out from the library). I've also started using it for school to keep track of what books I need to read this quarter.
Not only does it provide a way to keep track, but you can get recommendations. Give reviews, stars, and tag books. It is like your library catalog - only it does just what you want it to. Oh, and you can also follow people and see what they think of books and books that they add to their lists - a nice way to keep up with what your friends are reading.