I've been playing with an open-source free 3D graphics program, Blender, and I'm thinking it could be another good tool for webcomic making. It's pretty serious software, used for quality illustration and animation. (I suppose you could think of it as the CGI equivalent of Gimp.)

Installing it is as straightforward as for commercial software, at least in Windows, and there are Mac and Linux versions too. Not perfect - the Undo function's rather limited, so you need to be more than usually paranoid about regular saving. But it's certainly worth a look, and it could let you do stuff that would otherwise take a good chunk of budget.

Jim Chronister's Blender Basics downloadable classbook is the best introduction I've found yet.