An amazing X program by Jan Hubicka for drawing fractals. You've seen a million of these, but XaoS is different! It supports realtime zooming which really works, even on my puny old 166MHz Pentium (with no MMX).
Being able to pan and zoom around the Mandelbrot set (and 8 other equally cool fractals) at a good resolution interactively is an amazing experience. Pretty soon you reach the limit of your computer's precision, and have to start going back.
Support is also provided for Hubicka's aalib, for drawing in a terminal (or, nowadays, in an xterm or similar).