Inspired by Tym's adoption of GIMPShop, I finally downloaded X11, GIMP, and GIMPshop to try out. GIMP is an open-source application that does pretty much everything that Photoshop does, but without the price-tag. It's free - or, as the saying goes, "free, as in speech, and free, as in beer".
Here's a few photos from my first experiments with GIMP. First, a standard IR photo, from the set taken in the park. I didn't bother putting this photo through Photoshop because I thought I had enough shots of trees already (heh) but I ran it through GIMPshop to see whether I could get the same effects.
No discernable difference. These next two are interesting. I tried Terz's technique for making colour photographs look like infrared (see here, here and here), using GIMP. This wasn't really a test of GIMP's capabilities (I could've done the same thing in Photoshop) but more to see whether it was as easy to do the same things.
Exhibit A is a colour photo turned "IR":
Exhibit B is an IR photo, of the same subject:
(the same two photos can be see here, in their original form)
GIMP was just as usable as Photoshop for this task, though slower, and slightly clunkier (which is to be expected, for a new user). Ultimately, I still prefer the IR photo, but I'm going to be practising the other technique, because the Sony is getting old, and I don't think I'll be able to afford a replacement full-fledged IR camera anytime soon.
All in all, GIMP strikes me as a capable replacement for Photoshop. It seems to run slower than Photoshop (possibly because it has to run via the X11 environment on a Mac?) and the previews are noticeably less smooth, but if you're prepared to put up with a decrease in speed of workflow (which no doubt will be ameliorated by practice and familiarity), GIMP'll do the trick just as well. GIMPshop, by arranging all the menus in a Photoshop-like manner, also helps users like me transfer across with a minimum of fuss. And did I mention GIMP was free? = )