The Gimp is the GNU Image Manipulation Program, a sophisticated cross platform image editor. The Gimp is Free Software.
pandora plugin allows you to spread a series of photographs into a multi-layer image for easy editing.
Stitchpanorama plugin lets you pick control points and distort photographs to fit.
- The gimp doesn't currently support greater than 8 bits per pixel, ie. is doesn't support 16 bit per pixel high Dynamic Range images, though this is a target for future versions. However, there is a fork of the Gimp called Cinepaint that already supports 16 bits per pixel.
- The gimp doesn't support CMS color profiles for anything more than simple conversions.