GCompris - Educational Game for Children 2 to 10 years old.
Sum - Simple Maths Puzzle - How fast can you add two numbers?
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.
Simple Sudoku - Freeware Sudoku Puzzle Maker and Solver
I Know It - "I Know It" is an online game for practicing primary-level math concepts.
HoDoKu - HoDoKu is a sudoku helper written in Java and available in English and German.