
Photoshop (professional photography editor, free 7-day trial available otherwise subscription based (varies depending on app loadout, geographical location, use of academic credentials) - Photoshop is a professional photo editing system that photographers use. Powerful, with many tutorials on the web - great links for the main site.
It works on many operating systems, in many languages and is available for Linux, Mac OSX, Unix and Windows. Gimp (Free Software/Open Source) - The Gimp, or GNU Image Manipulation Program, is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring.