Scratch is a visual programming language for kids developed by the Lifelong Kindergarten group at MIT. You program by plugging blocks together and you can then share and comment on projects through the web site.
I heard about this listening to Nicholas Negroponte’s TED talk on “The vision behind One Laptop Per Child.” (A project to develop a $100 laptop for millions of kids to learn with around the world.) Scratch is the sort of free downloadable programming environment he imagines will be used on the OLPC by kids.