Scratch

Interest level: Grades 3-10 

From the about page on Scratch  :

"Scratch is a programming language that makes it easy to create your own interactive stories, animations, games, music, and art -- and share your creations on the web. As young people create and share Scratch projects, they learn important mathematical and computational ideas, while also learning to think creatively, reason systematically, and work collaboratively."

This program was created at the MIT Media Lab and was designed to introduce young people to programming.  Scratch uses an intuitive blocks system to help students construct their programs and is so easy that students (and teachers) can start using it in a matter of minutes. It's as simple as- if the blocks fit together then you have code that works. It they don't then you need to try again.

This program would be good for all kids and even adults to learn the basics of computer programming. Fundamental knowledge of how programming works can only of benefit to all who use computers, and who doesn't these days. In a school setting, Technology and Math teachers could easily add this to their curriculum. This would also make for a neat program in public libraries.