Scratch by MIT Media Lab ([0]) is a free Smalltalk-based visual
programming language and environment aimed at teaching programming.
The Scratch website lists 13,909,161 projects, all under the libre
CC BY-SA license ([1]). That's a *huge* amount of free programs, even if
a lot of them are a "remix", and not all of them are interactive!
Sadly, right now it uses Adobe AIR/Flash to play those programs and
animations in web browsers, and it seems that the official team doesn't
think porting the exporter to HTML5 has a high priority ([2]), and
although some independent developers started development, they stopped
at around 40% on May 8, 2015 ([3]).
I think it's good that so many schools, libraries and museums adopted a
free program, but most people use mobile devices nowadays, and Flash
doesn't work on all of them, so this could likely put an end to
Scratch's adoption, which will likely mean a proprietary program will be
used instead.
A free replacement for Scratch could be GDevelop ([4]), which is however
aimed at game development (but even if it's suited for professional use,
that doesn't mean it's harder to use).
I am not even a Scratch user, as I dismissed it back when it was under a
weird license and haven't really used it much after its GPLv2 release,
but I am a fan of Smalltalk, free software, free standards and
education, so I encourage everybody who has the skills or the funds to
contribute to this project, or at least spread around the word to
someone who might.
[0]: https://scratch.mit.edu/
[1]: https://scratch.mit.edu/info/faq/#remix
[2]: http://wiki.scratch.mit.edu/wiki/HTML5_Player
[3]: https://github.com/LLK/scratch-html5
[4]: http://www.compilgames.net/