What interests you? What is the real need?
For me, an on line bug tracking system worked wonders. I used a guestbook script as the basis, built in searching, sessions, authorisation etc.
Visit sourceforge and find projects which need people. or find projects which interest you and review how they've been put together and consider alternatives.
A project I'd like to do is a tracking of all our local train stations and local points of interest like parks. Visitors could add info about their local station.
I have small children so I'm always looking for interesting places to go on the train (they'd rather just sit on the train for hours). Think outside the square.