GIMP - GIMP is a multiplatform photo manipulation tool.
PixBuilder - Free Photo Editing Software for PC
Pixlr - Pixlr® makes it easy to transform everyday images into stunning works of art.
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.