LibraryThing

I like this because it allows you to make reading lists and get recommendations from other users that have the same taste in books as you. It also allows for discussions which could lead to... or maybe already has lead to... book club type things on this website. Everyone reads the same books then discussions are held on the various books. It gives you a chance to see what people think and add your own spin on things. I would have loved something like this for when I was in high school or college.