Math 

Everyday Math Resources comprehensive; by grade level
More Everyday Math Resources and other sites
Interactive Math Games K-6

Project Interactive Java based Math and Science
Activities done on-line; completed activities can be printed out as an assessment tool.  Grades 4-5
Rainforest Math - Interactive math activities for all levels
Go Figure - math-based puzzles, problems, and brainteasers
Interactive Math Activities
Math Wire - Monthly activities, pages - standards based for FREE!
Math Playground - Challenging games for students; logic puzzles; word problem resource...
Math Games Index by type