I have been spending many pleasant hours tracing the roots of Scratch all the way back to the 1970s and the pioneering work of Dr. Seymour Papert at the Massachusetts Institute of Technology Artificial Intelligence Lab. At that time Dr. Papert created the computer programming language for kids called Logo. He continued his work with Logo by teaming with the Lego® group to produce Lego® Logo and the current Mindstorms version of Lego® Logo. Scratch owes its snap-together blocks to the snap-together blocks of Lego®.
During my search I was thrilled to come across a paper presented at a 2010 conference on Constructionism by Brain Harvey and Jens Monig describing the extension they made to Scratch called Snap! Here's a link to their paper.
Their motivation was to add custom blocks to Scratch to make it a language suitable for High School Computer Science Advanced Placement and undergraduate introductory computer science classes.
Snap! Is free and runs in your browser. Here’s the link.
This is a screenshot of the Snap! browser. As you can see, there is a Scratch look to Snap!
A link to a reference manual is give near the bottom of the home page.