FractInt originated before floating point coprocessors became common and fast. It's named for the fact that it used integer math (fixed point) in its fractal calculations. It has evolved over time to include dozens of fractal types, scads of configuration options, and now even has abitrary precision math so you can zoom in farther than you'd ever want to.
The only really significant problem is that it still only works with 256 color images!
Originally written for DOS, there are ports available for Windows, Macintosh, Linux (and real Unix), and BeOS. (If you have Windows, just use the DOS version. Everything will be ok, really)
FractInt is currently above version 20.0 (how's that for a big version number?) available at www.fractint.org