Although they overlap in function, paint programs are really a different class of product from digital editors such as Photoshop and The Gimp. Paint programs are specifically designed to facilitate freehand drawing. They are ideal for sketching, painting, creating animations and special effects. Most folks have dabbled with Microsoft Paint (aka Paintbrush for Windows), which comes free with most versions of the operating system. In fact MS Paint is probably responsible for the generally bad reputation of paint programs. Early versions could only produce crude two dimensional drawings with jagged edges and no color gradients. Its free availability also encouraged untalented amateurs to produce works that could only be described as abysmal. Past reputation aside, the latest XP version of Microsoft Paint is actually a very capable and resource efficient product and deserves to be more widely used. In the hands of a good pixel artist it can give excellent results.
My favorite free paint program is however, not Microsoft Paint but Project DogWaffle. This is a commercial product but the developer offers an older version (V1.2) as freeware. The free version is surprisingly full featured. It has a comprehensive set of tools including fully customizable brushes, lots of filters and effects, multiple color selection options, a good set of color gradients, animation aids and more. On the downside the program runs slowly on older PCs, has limited layer support, has a clunky cut and paste mechanism and only handles .BMP and Targa files. Like all graphics editors, you’ll need to invest a bit of time to learn how best to use DogWaffle but those with artistic skills will find the effort well rewarded. Freeware, all Windows versions, 4.5MB.