
Blender - 3d dev
Gimp - image editing
Notepad++ - text editing (has an ArmA plugin that has some neat features, including one that will build the private definition), and syntax hilighting for programming languages
WinMerge - text (and binary, IIRC) comparison utility, for figuring out what is different about 2 files
Universal Extractor - will unzip almost anything, including most NSIS installers, and even some other kinds of installers
There may be better stuff out there, but these are the ones I've found to be very useful and powerful.
(Oh, and each of those are "portable" in that they have versions that you can put on a USB thumb drive and use without installing to a host PC.)