I've been using Real World Paint along with Gimp for a few years and I've been satisfied with it, making textures for a wide variety of engines. Recently I started using Inkscape for vector graphics (though RW Paint supports it and is easier to use).
I'm working on a main menu that'd be compatible with all non-BFG forks of idTech 4. I've already got it working, but it still is highly WIP. I'm writing it completely in Doom 3 guiscript focusing on functionality and minimizing required number of assets. I'll post a video once it's done or more functional.