A collection of online math activities. Ad-supported, but the ads are not (too) intrusive.
Several modules with step-by-step explorations of algebra concepts using the TI-83 / TI-84 calculators.
Nice collection of different web-based math tools and other resources
"Kids teaching kids". Videos produced by middle school students on different math concepts. These might serve as the basis for ideas for other teachers to use with their math classes.
British math resource site, "specialists in rich mathematics".
These are assessment resources for math teachers by Illinois standard and grade level.
NCTM online resource area. Includes online games and a library of lesson plans.
The main professional organization for teachers of mathematics. Lots of resources and publications. Includes an area on the implementation of the new common core standards.
Lesson plans, blogs, education news and other resources. Includes a search by state standards.
Broad collection of math resources for K-12, from the Alameda County Office of Education
Lots of stuff here, including an illustrated dictionary and nets for the Platonic solids (with tabs!)