Polish AFOL Paul "Sariel" Kmiec (who built this neat robot arm) (http://blog.makezine.com/archive/2010/03/pneumatic_lego_robotic_arm.html with facebook video) has created a Lego gear ratio calculator that lets you create complicated sets of gears and gives you speed, torque and ratios. I will also add the link at the top right of my blog page for future reference if you need to find it again another day.
Another tool lets you position two axles, then tells you what gears will work with that configuration. Lastly, you can choose which motor you're using and it will compute output speed and torque for that gearset.
Sariel is also an inspirational and innovative designer. A recent Sariel inspiration from his facebook wall. He is very good at explaining and showing how his designs work, eg tracked Leopard Tank problems and solutions. He has also been going through recording his preparation for LEGO Truck Trials. More about them in a future blog. Meanwhile look at this Tracked Land Rover inspired by the Inception movie.
He is a brilliant LEGO builder who I have followed over many years. Time to draw him to others attention if you don't already know of him.