too fun not to mention

If you already program, Scratch is going to live up to the name of the publisher--Lifelong Kindergarten. If you do not program I cannot think of a gentler place to start. You write programs by dragging and dropping colorful blocks that have individual comands on them. If you attempt to do something illegal, like drop a variable instead of a boolean into a conditional, the system patiently resists. If you can program, they give you a lot for free, and you can make highly visual little programs effortlessly.

If you do not program, there is also an attached community of thousands of people who have donated code of what they have been working on.  The code appears next to the window with the running program.  This, of course, makes it easy to play with and understand what individual lines do.  I even saw one guy who had written a compiler that runs console programs!  I'm not sure I'd want to write code in that language, but a neat display of Scratch's flexibility and power, as well as a window on compiler design.  More common are games and art projects of various kinds.

It is hard not to love and recommend Scratch, the language and the community.

A bit more about Anim8or.  For some idea of what is possible with it, you really ought to check out some You Tube videos on character animation by a guy called Soysauce.  The guy makes an Anime character, adds bones and poses it, and makes it look easy.  The only thing he did not cover, I think, is animation once you get your character.  Actually, the Soysauce has made something like 30 videos.  All the ones I have seen so far are a help and inspiration.