Go to http://www.gimp.org/ there are downloads for Win ,Mac0SX and Unix.
I've installed GIMP on Win XP and Mac 0SX with very few problems. The user interface is far better than years ago when I toyed around with it on a Win OS.
I've used CS3 recently and GIMP is tough competition for Adobe.
My new blawg header was a simple text in image I tossed together in GIMP.
It's free all the time no trials. It's open source code software.
Here's a picture of their mascot Wilber.
