librarything - YO

Librarything is really neat. One of our student workers at my last job introduced me to it. Honestly though, I don't use it with frequency, but it is really neat.
Some other librarything type sites are:

http://home.litminds.org/index.html

http://www.goodreads.com/

http://whatsonmybookshelf.com/

http://www.booktribes.com/

http://www.shelfari.com/