I first heard about BillMonk on the Ruby on Rails podcast. It's a really neat concept, helping you to track and share bills and things you might lend. The podcast is very interesting because it of course covers the rails aspects as well as the story behind the site itself.
You can use the site stand-alone, but it also has good integration with facebook - which is what I've been testing with some of my friends.
Well worth checking out if you share a house or often end up splitting meal or vacation bills with friends - or even just to keep track of movies or books you may lend out.