Well, I've been tracking F/OSS tools for "making" for a long time, and the list is getting rather unwieldy. If you're looking for anything specific, ask, and I'll see if I know of anything. Beyond that, a few favorites:
Pixel Prospector - a site that also maintains a huge list of resources for making games. Some of their lists overlap mine, but they're not exactly the same.
GIMP - photo manipulation and raster editor (kinda like Photoshop)