Try these fun sites: 

Crawford the Cat

 

Pattern Blocks

Primary Games

Scrabble

 

Concentration

(not all sites are accessible from school)

Eduplace Brain Teasers

The Kidz Page

Sudoku Mini

 Brain Boosters

Scholastic games 

Disney Kids' Island 

PBS Kids 

Wild & Wacky NGKids 

I Know That! (no need to register.  Click on "Maybe Later.")

How Stuff Works