nings

Nings are another good way to collaborate with other teachers.

You can find many different nings at the main ning website.

One really good ning is Making Curriculum Pop, and once you join, you can become a member of their math group. There is also a math group on the Classroom 2.0 ning. NCTM also has a ning.

Also consider creating your own ning for your school. You can make it private, and your members can create groups for their departments, clubs, etc.