GCompris - Educational Game for Children 2 to 10 years old.
Numito - Word and number puzzles game, similar to Countdown GameShow
Tux Paint - Tux Paint is a free, award-winning drawing program for children ages 3 to 12 (for example, preschool and K-6).
Wikids - An encyclopedia that talks to your kids
I Know It - "I Know It" is an online game for practicing primary-level math concepts.
Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.