I have to admit that I only found that one myself because I was curious what "premium apps" were included in Zorin OS Pro, and eventually found this list they provide of alternatives:
Yes. Illustrator is vector graphics so Inkscape is a more direct replacement, and Krita is raster so it’s closer to Gimp and Photoshop. One or both of those probably also have some vector, but not as much as with Inkscape/Illustrator. I think.
Unfortunately my knowledge only goes for the base program. But i've heard many saying that with the right time and extensions GIMP does replace Photoshop.
This is what I use if I can't use Illustrator - it's also got some terrific conversion tools. Currently the only app i know that can open and convert old Fireworks files.