The following sites are great for finding collaborators for your PBL units.

Nings to Join:

http://globaleducation.ning.com
http://ncssnetwork.ning.com
http://pblnet.ning.com/
http://www.classroom20.com/


Sites to list and find potential PBL projects:

http://www.cilc.org/c/community/collaboration_center.aspx Center for Interactive Learning and Collaboration
http://www.globalschoolnet.org/GSH/pr/index.cfm Global School Network
http://iearn.org/ International Education and Resource Network
http://www.epals.com/ Epals
http://www.twice.cc/projects.html Videoconferencing Collaborative Projects
http://www.magpi.net/Programs-Events MAGPI Programs
http://k20.internet2.edu/ Internet K20 Project
http://www.tigweb.org/ Taking it Global
http://teachersconnecting.com Teachers Connecting

Twitter:

If you don't already have a Twitter account, get one now! This is a great way to create a personal learning network and to find collaborators.
http://twitter.com