Open Source Software
Open source software is computer software whose source code is available under a copyright license that permits users to study, change, and improve the software, and to freely distribute it to potential users. Although I am listing several open source software choices on this page, I will not guarantee the safety of the downloads.
Tux Paint Tux Paint is a free, award winning drawing program for children ages 3 to 12 (for example, preschool and K-6). It combines an easy-to-use interface, fun sound effects, and an encouraging cartoon mascot who guides children as they use the program.
Tux Math An educational math tutorial game starring Tux, the Linux Penguin. Based on the classic arcade game "Missile Command," Tux must defend his cities. In this case, though, he must do it by solving math problems. 
Tux Math Scrabble TuxMathScrabble is a math version of the classic word game "Scrabble" (Trademark of Hasbro,Inc) which challenges kids to construct compound equations and to consider multiple abstract possibilities. There are four skill levels for practice, from basic addition with small numbers, through multiplication and division with larger numbers. The game can be played by 0, 1 or 2 human players.
Tux Typing TuxTyping is an educational typing tutorial game for children starring Tux, the Linux Penguin.
Multiplication Station Multiplication Station will teach your child to add, subtract and multiply.
GCompris GCompris is an educational software suite comprising of numerous activities for children aged 2 to 10. Some of the activities are game orientated, but nonetheless still educational.
Childsplay Childsplay provides memory activities that are fun to play and at the same time learn sounds, images, letters and numbers. It provides activities that train the child to use the mouse and keyboard and also pure game activities like puzzles, pong, pacman and billiards. 
Celestia The free space simulation software that lets you explore our universe in three dimensions. 
Stellarium Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope. It is being used in planetarium projectors. Just set your coordinates and go. 
Scratch Scratch is a new programming language that makes it easy to create your own interactive stories, animations, games, music, and art--and share your creations on the web. Scratch is designed to help young people (ages 8 and up) develop 21st century learning skills. As they create and share Scratch projects, young people learn important mathematical and computational ideas, while also learning to think creatively, reason systematically, and work collaboratively. 
Alice Alice is an innovative 3D programming environment that makes it easy to create an animation for telling a story, playing an interactive game, or a video to share on the web. Alice is a teaching tool for introductory computing. It uses 3D graphics and a drag-and-drop interface to facilitate a more engaging, less frustrating first programming experience.
Free Reading FreeReading is an open-source instructional program that helps educators teach early literacy. The program contains a 40-week scope and sequence of phonological awareness and phonics activities that can support and supplement a typical kindergarten or first grade core or basal program.