Actually, I just went through that list, removed anything that didn't have critical functionality, and then counted how many "Yes" entries each software had and sorted descending.
The first non-proprietary package comes in at 6th, and is
Krita - not one I've heard of before, it's "digital painting and illustration software", so isn't focused on photographic features.
Annoyingly, that's not the only catch, there's also this one:
quote:
For Windows and OSX we currently do not provide binary installers, and building on Windows or OSX is difficult. (Which is why we don't have binaries for you.)
:(
The next free one on the list is Paint.NET, which is shit.
After that comes Pixia and mtPaint which both look even worse.