This is not meant to be a comprehensive list! Many of the sites below have much more up-to-date lists of PER groups. Below are just a few with which/whom we have personal connections here at CU.
Iowa State :: They have a nice (and probably the most comprehensive and up-to-date) set of links to other PER groups there, too.
University of Maryland :: Lots of useful pages, including most of their publications, "challenge problems", Maryland Physics Expectations Survey, etc. ) They have a number of more detailed "lists of links" than we could possibly provide here, see for example their list of PER homepages, curriculum materials, and online papers.
University of Minnesota :: Their main focus is on Context Rich Problems and Collaborative Learning.
University of Washington :: They started this game, with research into student mis/preconceptions. The source of the recitation "tutorial" method.
PER Central :: Hosts a collection of resrouces and information for physics education research nation-wide.
North Carolina State :: SCALE-UP, the modified introductory classroom approach at UNC (Bob Beichner's group).
And more: e.g. Arizona State University.
Richard Hake's research in physics education page (with many useful links and references)
Collaborative Learning :: Detailed instructions specifically targeted to forming and using groups in classrooms.
Classroom Assessment :: Detailed instructions regarding a variety of different tools to use in the classroom. A great place to learn about a variety of novel classroom techniques!
SALG :: This is the place to set up a survey of student opinions regarding your teaching methods. It's a nice tool, quick and easy to set up for you and for your students.
TextRev :: Similar to SALG but even quicker and simpler, this site is dedicated purely to setting you up with a student survey regarding your textbook.
Do you know of a useful link our colleagues might be interested in? Let me know and I'll add it in. firstname.lastname@example.org