I recently stumbled upon Good Reads when I saw it on someone's facebook profile and thought " Oh! How neat!". I am trying to read more, and having a lot of recommendations saved online makes it so much easier! You can see what your friends have read, what they are currently reading and how they felt about those books. Also you can see strangers reviews. It's a great way to keep track of all that you have read!
They also make easily transferable badges for blogs, myspace, facebook etc. Look to the right margin toward the bottom to see mine!