My favourite photo editing software happen to be open source. I use the GNU Image Manipulation Program (GIMP) for photo retouching most of the time, though it can used for image composition and image authoring. However, after having been using Photoshop for a long time, it took me quite sometime to used to it.
Today I learnt about another open source photoeditor called Gimpshop (8154KB). This powerful tool looks a lot like Photoshop with several effects and filters. As the name suggest, it is based on GIMP, but has more features plus that Photoshop feel. Don't forget its' totally free.