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Teaching your kids how to write computer programs is an essay by Marshall Brain, creator of HowStuffWorks.com. It includes a lot of links.
Scratch -- Free Kids Programming Language from MIT
Guido van Robot: Guido van Robot, or GvR for short, is a programming language and free software application designed to introduce beginners to the fundamentals of programming. GvR runs on Windows, Macintosh, and GNU/Linux, in a variety of languages! It's great in both the classroom and the home as a way of introducing people to the basic concepts of programming.
Alice: Free teaching tool designed to be a student's first exposure to object-oriented programming. It allows students to learn fundamental programming concepts in the context of creating animated movies and simple video games in 3-D.
How to Think Like a Computer Scientist: Learning with Python 2nd Edition: Open book project
by Jeffrey Elkner, Allen B. Downey, and Chris Meyers