These are some pictures of a robotic platform that I built using the plans for the adder/subtracter found on Leo's Lego . While I'm using the basic ideas from the design, I've changed it in many ways.
My design has all of the gears in one layer (mostly because this allowed me to reduce the number of gears required). One bad apart about this is that the inputs are now on opposite sides of the adder/subtractor, so the final robotic platform looks a little funny (it has one motor in the front and one in the rear). To make my platform work I also had to make two 5-unit axles (by cutting one 10-unit axle in half).
The platform allows you the robot to get the normal four motions (forward, reverse, spin left, spin right) with a perfectly straight forward and reverse. One of the motors is used to move the robot forward and backwards. To turn the robot left and right you use the other motor.
Note that the robot as shown isn't very strong. I added a bunch of extra support pieces to it after taking these pictures. I haven't taken pictures of the robot built as it is, because it would be hard to see how the design worked.
The top of the unit:
The bottom of the unit:
The bottom of the unit, closeup on the drivetrain: