Once in a while a web company comes along and does a fantastic job solving a very particular day-to-day issue. Not trying to become the next big web thing or to dethrone Google or anything like that. Just solving one issue and doing a great job of it.
My 2006 favorite in this category has got to be BillMonk. They explain the service better than I could:
Friends cover for each other all the time. We make it easy to manage roommate expenses, split restaurant bills, settle up after a group vacation, and record simple debts.
They've also recently added support for tracking stuff (books, DVD's, whatever) you borrow or lend:
Friends lend books, movies, and other stuff. Make us your librarian and we'll make sure you know where everything is. Friends can even share their collections and browse the group lending library!
They do it all with a great simple interface and have all the right features (SMS support for when you don't have access to a PC, shared bill support, etc, etc) with none of the useless features they could probably bloat it with. The only feature I couldn't find is the payment option!... I'd happily pay a few $'s a year to help support this great effort.
Go and take their product tour or sign up now.
TechCrunch has a more comprehensive review of BillMonk here.

Hi, I'm one of the co-founders. Thanks for the nice comments, we really appreciate hearing that our service is useful! Adding payments integration so you can settle up via BillMonk is our #1 focus and will be added in a matter of weeks. We are considering a business plan of taking a teensy cut of such settlements, or adding new premium features which are available for a low subscription fee (never fear, everything that you can do for free today will remain free).
Posted by: Chuck Groom | April 25, 2006 at 06:38 PM