Aseprite calls itself "source available" now. Their license has a "no redistribute" clause, so if want to use it and can't compile it yourself, you have to buy it.

I spent about an hour trying to compile it on Windows a few months ago, couldn't get it working. Can't remember why, maybe most of that time was installing requirements.

Seems like a bit of a perverse incentive.

They were GPLv2 for a while but changed course. I'd use the fork going forward:

https://github.com/LibreSprite/LibreSprite

From memory, one of their dependencies (maybe skia?) was a gigantic pain to build. I also wasn't very familiar with ninja, and couldn't debug some of the ninja output from cmake.

Doesn't really matter because I already had a license, but it would have been interesting to play around with the internals.