I have been a long-time user of the music synthesis program Csound. It is free-to- use and you can look at its source code but it's not Open Source. It uses an MIT license with a clause that restricts its use to academic and research purposes.
I have been secretly working on converting its 500+ reference manual to DocBook. But since it's not Open Source, it's hard for me to get motivated enough to finish. I keep reminding myself that I'm making it "more free".
Richard Dobson, maintainer of Csound's phase vocoder code, has been looking for loopholes so that they can steal GPL code. To me, this is plain wrong. They need to make Csound Open Source instead.
Of course, this will never happen. Not until some people at MIT pick up the clue phone. If you think going up against Microsoft is hard, try the insular world of academia.