Math Playground has all kinds of online math games to play.
In their own words: "Welcome to Math Playground, an action-packed site for elementary and middle school students. Practice your math skills, play a logic game :and have some fun!"
They have thinking blocks, manipulatives, worksheets, flashcards, multiplayer learning games, word problems, math videos, a computation section and a math apprentice section. And as far as I can see, all free!
This has been linked up at