I love pizza so made a pizza rating website with posts, profiles, likes, comments, and a map. We pestered all our friends to use it but couldn't get anyone else on it. Apart from a new pizzeria in Copenhagen which posted an advertising review ;P

A service which scrapes tennis courts around London and notifies you if a slot you want becomes available. We've had over 150 players sign up, >2000 notifications sent, and received £30 in donations!