GIMP

GIMP (the GNU Image Manipulation Program)

GIMP is a free software raster graphics editor. It isprimarily employed as an image retouching and editing tool. In addition to offering freeform drawing, GIMP can accomplish essential image workflow steps such as resizing, editing, and cropping photos, combining multiple images, and converting between different image formats. GIMP can also be used to create basic animated images in the GIF format. At present GIMP is entirely suitable for amateur or professional work with images intended for viewing on monitors and printing on inkjet printers; GIMP does not yet offer the CMYK separation and color management functionality which is essential for prepress work. The product vision for GIMP is to become a high-end graphics application for the editing and creation of original images, icons, graphical elements of web pages and art for user interface elements.

It i also sometimes compared to Photoshop and as a good alternative to it, though maybe not so much level in profesional ground.