Um, isn't this just an exact clone of Scratch? http://scratch.mit.edu/

From their site:

It is an extended reimplementation of Scratch (a project of the Lifelong Kindergarten Group at the MIT Media Lab) that allows you to Build Your Own Blocks. It also features first class lists, first class procedures, and continuations. These added capabilities make it suitable for a serious introduction to computer science for high school or college students.

I did not find this at all intuitive. Possibly because I have never worked with Scratch before. No idea what I was supposed to be doing and no obvious forms of help.

Documentation for Snap! is underway...

There is some help on http://snap.berkeley.edu (without the /run suffix)

Check out the Reference Manual, which is also a concise introduction to the IDE: http://snap.berkeley.edu/SnapManual.pdf

and some introductory tutorials on Youtube:

About "Build Your Own Block: http://www.youtube.com/watch?v=Bbl2fh3igQ4

And on how to add inputs to them: http://www.youtube.com/watch?v=hZC2tDVYWBo

Also, there are helpscreens in the context menu for each block

Awesome, makes programming accessible to the uninitiated. I was able to create a simple event loop with asteroid like mechanics in a few minutes.