GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software suitable for such tasks as photo retouching, image composition and image authoring. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, a image format converter, etc.
XPaint is an easy-to-use image editor which supports many standard and less standard paint procedures that demanding non-professional users would expect. It includes several advanced features like image processing algorithms, programmable C scripts, simultaneous editing of multiple images, and supports a wide variety of image formats.
Grafx2 is a bitmap paint program that allows you to draw in more than 60 video resolutions (from 320x200 to 1024x768, including most of the standard Amiga resolutions: 320x256, 320x512, 640x256, 640x512, and more, provided your video card knows how to handle them). Its layout is not very different from the famous Deluxe Paint or Brilliance, so it will be quite easy to handle it if you know at least one of these programs.
Pixel is an application for editing, retouching, manipulating, and animating RGB, CMYK, and HDR images. It was formerly known as Pixel32. It is available for Windows, Linux, Linspire, Mac OS X, BeOS, Zeta, QNX, MorphOS, FreeBSD, eComStation, OS/2, SkyOS and even old plain DOS, for both x86 and PowerPC architectures.
CinePaint is a deep paint image retouching tool that supports higher color fidelity than ordinary painting tools. CinePaint is used to retouch feature films and for professional photography. CinePaint opens high fidelity image file formats such as DPX, 16-bit TIFF, and OpenEXR, and conventional formats like JPEG and PNG. It has a flipbook for movie playback of image sequences in RAM. It supports 8-bit, 16-bit, and 32-bit color channels, HDR, and CMS.
SVGSlice is a simple tool which generates chopped up images from Inkscape SVG drawings. To figure out where to cut, SVGSlice can use Inkscape's built in guides (for simpler, grid-like cuts), or it can use a specially named "slices" layer that contains rectangles that mark areas to slice. Demo files showing both approaches are included.