Windows/DOS
Sterling-Ware (Windows, Freeware)
Much like Grafzvizion 24bit color, fast zooming.
GrafZViZion (Windows, Freeware)
Very Fast Zooming, My pick. 24bit color
Flarium24 (Windows, Freeware)
Very good program lots of options, goes up to 24bit color, various sizes.
Tiera-Zon (Windows, Freeware)
Made by the same people who made Flarium24, much faster.  Also 24bit color and many sizes, makes cool movies. You might also want some sample Source files.
Fractint (Dos, Windows, Freeware)
Lots of pre-made fractal demos.
Fractal Extreme (Windows, Shareware)
Ok program, makes neat zoom movies.
Ultra Fractal (DOS, Shareware)
Another ok fractal program.

Other  Fractal Pages
Sprott's Fractal Gallery
Fractal World
Julian's Fractal Page