The Gimp: free and good.
GIMP stands for "General Image Manipulation Program". Although it has an odd name, "The Gimp" is a very powerful program for editing digital photos. The Gimp is quality open-source software, originally written by Spencer Kimball and Peter Mattis and now developed and maintained by hundreds of talented programmers from around the world.

cross-platform, free, AND good.
Photonhead online samples and tutorials use The Gimp.

  • feature packed
  • free to use open source software
  • available for Windows, Mac, and Unix
The Gimp for Windows

Downloads:

Download GTK 2.2.4
gtk+-2.2.4-20040124-setup.exe
3.55 Megabytes

Required runtime files. (Install first)

Download The Gimp 2.0
gimp-2.0.0-i586-setup-1.exe
11.4 Megabytes

The Gimp program. (Install second)

more links:

The Gimp for Windows Homepage

 

The Gimp for Mac OSX

more links:

The Gimp for MacOSX Homepage

MacGimp.com
community portal for Mac Gimp users.

 

The Gimp for Unix

more links:

The Gimp for Unix Homepage