Photoshop is the top professional image editing / photo retouching program, but it is expensive. There are some cheap and free alternatives available:
Adobe Photoshop Express (online image editing, requires sign up at Adobe’s site) http://www.photoshop.com/tools/overview
The GIMP is open source software http://www.gimp.org/
GIMP Installer for Windows http://gimp-win.sourceforge.net/
GIMP Installer for Macintosh http://gimp.lisanet.de/Website/News/News.html
Seashore for Mac OSX http://seashore.sourceforge.net/The_Seashore_Project/About.html
Photoscape for Windows http://download.cnet.com/PhotoScape/3000-2192_4-10703122.html?tag=mncol;1
Google Picasa http://picasa.google.com/
Picasa Mac
List of free image editors at C|Net download.com
Windows Digital Photo Software
Windows 7 Image Editors
Mac OSX