2007 brought an increased awareness of the many interesting literary websites and communities online that allow readers all over the world to share their book recommendations, reviews and even spoilers.
I am a newfound Library Thing convert. It's an online service to help people catalog their books. You can access your catalog from anywhere (seriously, you can get it from your cell). It also connects people with the same books and comes up with suggestions for what to read next. While most people spend their time cataloging their iTunes library this site is ideal for all those book worms out there.
A free account allows you to catalog up to 200 books, while a paid account allows you to catalog any number of books. You'll never have to wonder "have I read that?!" again.