Goodreads. Basically, you have a profile where you can make a list of all the books you've read, give them reviews, etc. On mine, I've made a separate 2010 Reading List (that I need to get cranking on), and other people can see what I intend to read this year!
It IS a protected site, though, so you can approve certain users to be your "friend" on Goodreads.
I have definitely found it easy to get lost in this site simply because it gave me a chance to reminisce on all of the great classics and comedies that I've read over the years. You find yourself searching for every book you can think of to add to your list. I love it!
Anyone have any books on their list they would recommend?