Activities below by subject many can be used for multiple grades.
K-8 Multiple Topics http://www.funbrain.com/
Any Age Create a Comic
StoryPlace - Digital Stories in English and Spanish http://www.storyplace.org/storyplace.asp
Variety of Activities for preschool - 8th Grade Professor Garfield
All Subjects, Variety of Grade Levels BrainPop
Variety of Subjects HippoCampus