GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. The terms of usage and rules about copying are clearly listed in the GNU General Public License.
For additional information, please refer to: GIMP: LSU Overview.
imgSeek is a photo collection manager and viewer with content-based search and many other features. The query is expressed either as a rough sketch painted by the user or as another image you supply (or an image in your collection).
For additional information, please refer to: imgSeek: LSU Overview.
Krita Studio is the powerful painting application for professional artists, VFX and game studios. The uniquely innovative brush engines, the intuitive and fun interface and adherence to industry standards for pipeline integration, guarantee that Krita Studio supports your workflow and helps you get the most out of your ideas. It is a good alternative if you want photoshop but can't afford it. It is open source and seems aimed at game makers and artists.