Welcome to the distribution center for BYOB (Build Your Own Blocks), an advanced offshoot of Scratch, a visual programming language primarily for kids from the Lifelong Kindergarten Group at the MIT Media Lab. This version, developed by Jens Mönig with design input and documentation from Brian Harvey, is an attempt to extend the brilliant accessibility of Scratch to somewhat older users—in particular, non-CS-major computer science students—without becoming inaccessible to its original audience.
Great, Scratch has proven to be the best tool for getting kids started on programming, computational thinking, etc., with a "low floor," and with BYOB, you should be able to keep going with a "high ceiling."
This is and will remain free software, subject to the MIT Scratch license that sets conditions for distribution of modified versions such as BYOB.
That is, BYOB is free software, except insofar as it is required to retain a non-free license which is technically incompatible with all other free software and free software distribution channels, as will be the case for all Scratch-derived projects.