Detha finds scratch the best place to start young kids, outlines some of its higher order limitations expertly, rejects game maker because its Windows only, finds eToys inpenetrable, mentions Bots and looks at how to make python programming more accessible. Also discusses Quartz Composer which I've never used. It's a great blog.
I like his succinct outline of scratch limitations:
It also has some pretty severe limitations: no user-defined blocks, no return values, no file interaction (so no high scores), no network interaction, no dynamic object creation, the program cannot draw on sprites (only on the background), no string variables or any real string handling. It is a great environment for learning to think creatively within its constraints, but my kids also bump up against its limits pretty quickly.