Yep, Gimp! When I was well-versed in it's ways. I used to use GIMP for everything. Most of my old digital art was all done in GIMP, I'm spoiledw ith SAI since I have NO CLUE how to use GIMP any more. XD I'm so used to the pressure sensitivity with my tablet pen and GIMP doesn't have it.
So was I, actually. I haven't used for for like a year, though! And my tablet DOES work with the pressure sensitivity, but it's just such a limited tool compared to SAI. All of the old digital art on mine was done in gimp as well, some of it done with A MOUSE in gimp... so I know where you're coming from. XD