Quick Personal Intro. I come from a multi-media background. I was also a GM who would go crazy with the preping material for my games. Some of them survived and can be found in my deviant account. My Thesis was a RPG based in the 19C Philippines. I am also a trained 3d Artist but I never got to seriously apply my skills after my apprenticeship because my parents pirated me for the family business.
Open Office. I use Open Office. and I use it even when even when I have access to Microsoft Office at work. Back when i started using it, it was still better than the older MS office tool I had to contend with (around 3 years ago). I got introduced to this because of the guys in the sj forums who insisted in it. GMs who spend HOURS on writing and preping handouts will find this an easy learn. Probably 5 hours of practice and its like you were never using MS.
GIMP. Graphic Image Manipulations Program is basically Photoshop that is Open Source (free and no worry about . if you have problems with the site, you can also download it here. The key difference between GIMP and Photoshop is the filters, which isn't that plenty. Of course you will need to learn a different Graphic User Interface but thats not much of an obstacle. In probably 10 hours (a month of free time doodling) of being able to use most of the key features, you'll get it.
If you can get your hands on cheap 12" tablets for $40 and practice drawing on the program, it will take roughly 7 hours of practice to get a complete hang of it. If you want to learn to draw, there are a lot of youtube video tutorials, help in deviant art, and it takes roughly 400 hours to do learn to draw close enough to professional level if you can get someone who can honestly critique you and has an art background to direct you.
Blender. What does a GM need a 3d Graphics Program for? Well if your a GM with a lot of spare time, there are many uses for Blender. When i was a student, this would have been really usefull, but I only discovered it when I was apprenticing 3d Animation (which was for the use of 3ds Max for character animation). One of the best uses for this is Realistic Mapping Effects. 3d programs do the job of map making better because of the special rendering tools you have. Type in Deviant art 3d Landscapes. Blender is very usefull if your the type of GM who is obsessed enough about your own setting to build material for it. The tutorials available for blender are plenty and easy to find. When i made my thesis part of my business plan was to make 19C philippines 3d map to be used as a Computer Game setting, a mix of Sid Meyers Pirates, Sim City, Merchanters, Mercenaries, and Private Armies. Who knows, maybe if I get my act together it still can be possible, 3d artist services are going down.
Scribus. GMs who can be very obsessed with preparing material can find scribus a good substitute for Adobe InDesign. It is a PDF publishing and layouting program. Maybe in two years , there will be enough of an ebook market because of ereaders that the adventure you spend 6-10 hours a month prepping will have huge amoung of buyers to be worth your while. Maybe you can work with some artistically inclined friends together with some art.