One is a magic wand that can be set to pick up only those colors you specify, a useful feature. There's a FatBits editor and a Goodies menu with resize, invert, and colorize options. Although confined to 256 colors, Matt Paint supports adaptive palettes. The program does have some notable limitations. You can only open one document at a time.
Tools sometimes misbehave, and switching between patterns and solid colors is a headache.
Matt Paint is starting to show its age, but the program has more features than several others, and the Undo button on the tools palette should be a standard. Supports PICT and JPEG-compressed PICT formats only. Created for scientific imaging, NIH Image has long been cherished by artists and others on limited budgets. Its most notable feature is the number of convolutions and transformations it can perform. In addition to standards like smooth and sharpen, Image does complex image math, combining multiple images in a variety of ways.