I Know It - "I Know It" is an online game for practicing primary-level math concepts.
GCompris - Educational Game for Children 2 to 10 years old.
Numbily - Free Math Game - Train your brain in a easy free math game! Improve your math skills!
Tux Paint - Tux Paint is a free, award-winning drawing program for children ages 3 to 12 (for example, preschool and K-6).
childsplay - Childsplay is a collection of educational activities for young children and runs on Windows, OSX, and Linux. Childsplay can be used at home, kindergartens and pre-schools.
NumberDrop - A uniquely challenging math puzzle game.