Scratch is a great environment for learning to program produced for free and to a very high standard by the Kindergarden Lifelong Learning Group at MIT. This is the home of Logo and many other computing frameworks for children.
Scratch is aimed at 8-12 year old children although it can be used to teach programming concepts to younger children and adults. The design environment runs on PCs, Apple Macs and Linux and the final projects can be shared and run on most web browsers.
