I found this site today in one of my news feeds. The site lets you select various gears and then the ratios will be calculated which will then display the end torque and speed of the axle. You can also select the holes in the bricks where the axels are and the site will show you which gears will work and the information about that gear combination.
The site really wants to make me play with my LEGOs again.
Edit: Make the link an actual link.