Having an iPhone I find myself taking alot more spontaneous photographs. By far I am not a professional photographer, but I like to think I have an eye for a picture. At my old work place I use to use Macromedia Fireworks for touching up photographs or image manipulation/creation. When leaving I started to hunt about to see if there were any good free opensource alternatives and I came across Paint.Net. A year or so later I thought I would revisit this area to see if there are any others out there worthy of using.
Virtual Studio
Chasys Draw IES
Paint.NET
GIMP
Fotografix
Obviously for serious photographers they need something like photoshop or lightroom but for casual shooters these kind of simpler apps are terrific and help you create nice imagery.