Gimp (Open Source)

[Gimp] is a raster editor, which means that it performs operations directly on the pixels that make up the image, and not a vector editor. Other (proprietary) raster editors include Adobe Photoshop, Jasc Paintshop Pro and the humble Microsoft Paint. An alternative free editor is the KOffice project, Krita. Users wanting to edit photographs will certainly want a raster editor like GIMP. (source: http://www.gimp.org/docs/userfaq.html#What)