Math

AAA Math
A+Math
Cool Math
Learning Planet Games
Logic Games
Math Dictionary
Math Manipulatives 

Language Arts

Grammar Gorillas
Kiz Clu
Lil Fingers
Mad Libs
Mighty Book
Poetry Pickle
Starfall

Science

Bill NYE
Franklin Institute
Nasa Kids
Periodic Table

Research

World Book Online
Annals of American History

Social Studies

50 States
Atlapedia
Ben's Guide
Fact Monster World of Geography
On This Date
US History
Time for Kids