IrfanView : The best graphics viewer in existence. Also comes with basic editing features, plugin capabilities and audio and video capabilities.
DotColor : Provides RGB and HEX color codes for anything you can see on your monitor.
The GIMP : Recommended by Pistos, a GeoShell user, who said, "The best free alternative to PhotoShop, Corel Draw, Paint Shop Pro, etc."
MWSnap : Recommended by Pistos, a GeoShell user, who said, "A good screen capture utility. You can marquee sub-sections of the screen to capture."
Paint.NET : I've been using this at work. It's a basic graphics editor that is not as complete as The GIMP or Photoshop, but has more functionality that Irfanview. I use it when I need to make changes to images (or create them from scratch) and still use Irfanview for just viewing images.