Scratch (read story)

A free programming tool that allows anyone to create their own animated stories, video games and interactive artworks has been developed.

Primarily aimed at children, Scratch does not require prior knowledge of complex computer languages.

Instead, it uses a simple graphical interface that allows programs to be assembled like building blocks.

Also check out Gamemaker (http://www.yoyogames.com/make) (thanks Keith) which seems to be more full-featured, but I don’t know how easy it is to use.  I’m thinking of using some of these types of tools to introduce my boys to computer programming.