Gimp

The Gimp for Windows is actually pretty nice, if that's what you're used to. I've been using it almost exclusively for about 7-8 months now, and generally prefer it to photoshop for most things. The main difference is the fact that almost everything is done by right-clicking on the picture you're working on, rather than drop menus, as there is no MDI. There also is a plugin for it that is supposed to let you use photoshop plugins, though I haven't tried this yet.

Problems I've encountered are that the menus from the main window don't go away if you don't select something, and are sometimes hard to get to go away. This has been improved in a later "unnoficial" release (link below). Also the text isn't as full featured as on photoshop (i.e. bending/curving/wrapping text, etc.), and some of the plugins crash semi-randomly.

http://www.gimp.org/~tml/gimp/win32/new-downloads.html

^^^It says for developers and power users, but I'm neither and have had very few problems installing/using this version. It does make the menus a lot more usable and some of the plugins less buggy. There are probably other bugs I haven't his yet but that's the fun of free software :-)