You can see one chassis pretty well in this vid:
It's a dollar store erector set knockoff bulldozer kit with 2 servos modified for continuous rotation. That seems to be the easiest route to turning wheels/tracks etc.
The vid in the first post is a Tamiya dual gearbox and Tamiya track set that I rigged up with another cheap-o erector set.
Here's a couple of bad pics:
The second is a board with an L298 driver which seems to work well with the Tamiya box. It may not be the best choice but it was fun to build
The gearbox is a little model that you assemble and the track set is just a bunch of parts. I got them both from Sparkfun.
Yea, bots and stuff that rolls around under power are cool to mess with
That board turned out to be pretty large and I think I'm going to try to duplicate it only smaller. Maybe with a home brew PCB
Here's a link to a thread about my L298 adventures:http://arduino.cc/forum/index.php/topic,116935.0.html