TAMIYA twin-motor gearbox
This is a very nice little package. You have to assemble yourself, and there are 3 ways.
You want to assemble in "C"-mode, ratio 203:1
The A & B setups, 58:1 are really fun because they are fast etc.. until you try to make your robot use it. It is simply not strong enough for the fast-changing pulses etc from a PIC-controller, it is useless because it takes so much time to accelerate, and many setups will not ever get up to speed at all.
I strongly recommend that you buy some TAMIYA wheels or belt-tracks to accomplish, and also other parts in this series from TAMIYA, the plates, sticks etc.
You should check this link: http://www.pololu.com/docs/0J11
Here is a robot driving one of these http://letsmakerobots.com/node/386