Introducing a Child to Constructive Computer Use?

trevorgensch asks: "I have a young boy, about 6 years old, who is starting to take an interest in the computer I seem to spend too much time at lately. Lots of Slashdot readers out there must have had experiences with their young child wanting to learn more. I am all for it! But where to start? He has had a bit of experience with the Internet and children's sites and official sites of Pixar and Disney movies, but he wants more. Any pointers?"