-IMSA Center for Problem-Based Learning
The Illinois Mathematics and Science Academy is a school for forward thinking faculty and students. Problem-based learning is just one of the many techniques they employ at IMSA.
-MCLI's Problem-Based Learning
Maricopa Center for Learning and Instruction deals with quite a bit of Problem-based Learning, and most of our research has come from here.
-SCORE Internet Classrooms
SCORE gives a breakdown of Problem-based Learning, including possible shortcomings on their site.
-Ideas for PBL, URLs
This is a reference page containing several links to subject-specific PBL sites.
-University of Delaware's PBL Site
UD has a comprehensive list of books, articles, syllabi, and problems about PBL, which they also refer to as a Pathway to Better Learning.