All grades, math and science
http://www.sheppardsoftware.com/
Learner Interactive Spelling Bee
All grades
http://www.learner.org/interactives/spelling/
PBS Kids Interactive Board Activties
Primary Grades
http://pbskids.org/whiteboard/
Kids Math Games Online (Board Games)
3rd and up
http://www.kidsmathgamesonline.com/boardgames.html