RelayForLife
Every year in various locations around the country there is a large fund raising event to raise money for the Cancer Society. The event involves teams of people getting sponsored, and the Relay For Life web site makes it all possible.
Team captains can invite people to join the team, and when those people join up each person can use the site to send out requests for sponsorship. Sponsors can respond to those invitations and the amount of money they sponsor is tracked against each participant, team and region. An administration area allows the Cancer Society to maintain contact with team captains and manage the communication of the event.
It was originally developed by someone else but I have been maintaining it since early 2005.