I just recently found out about
MLCAD from Jon Lawton. It's a free CAD system for Legos, and is very fun. It's included in the full distribution of
LDraw, which is very cool because it requires a few other components from LDraw (but which aren't included, if you just download MLCAD). It includes models for practically all Lego products, including the Technic series gears, motors, axles, etc. They even have a model Lego RCX.
Anyway, it's not really
useful, but I thought it was worth mentioning...