This page contains projects the ideas of which (either entirely or partially) could be duplicated in certain classrooms:

One in 8 Million - the stories of regular people who live in New York City
PhotoWalk - get students together, with their cameras, taking many pictures of the same event, location, etc. Upload them to a site and/or create something from that.
Blog Book - have students write a book from posts entered by different people.
NASA Space Math
The Place Plus Memory Project - Recreating things from the past that made an impact on our lives.
Design a Shelter - or whatever, using Google Sketchup
Kiva - study countries and businesses and loan money ($25 minimum)
Dear Daddy
Internet Sites That Support PBL
60 Second Recap