I found a few more character generators:
Hero Machine -- nice-looking characters, but too detailed to look good scaled down to 100 pixels. (Aim for simpler accessories, and maybe if you crop for just the torso....)
South Park
Kids Next Door (They can get a lot sillier than this.)
PowerPuff Girls and Guys!
DookyWeb
Late additions (Fri 2004/Apr/9):
Mini-Mizer LEGO
The LEGO page also explains how to capture and save images from the screen (Windows and Mac).
¡Mucha Lucha! You too can be a Mexican professional wrestler.