After reading about the use of cordless drill motors in robots, I decided to attempt a three-wheeled platform inspired by Missouri S&T 'bots Aluminator and Jomegatron of my own. For $60 at Harbor Freight I came home with two 10" wheels, a swivel caster, and two 18V cordless drills. I promptly disassembled the drills to harvest the motor and gearbox, desoldered the switches, and attached longer wires to them. Each motor/gearbox is around 8" long (including chuck, which I am leaving on) so my 'bot will likely be 18" wide (adding 2" in between the motors for wiring). The caster will be centered at the rear with the battery (either paralleling the two 18V NiCd packs or a 12V SLA) rests in the middle, keeping the center of gravity in the middle of the robot. I want it to be wide enough for a laptop to sit on top, and also want to add a camera mast with USB webcam for remote driving and maybe autonomous driving.
The frame is all going to be made out of wood, with a plywood base holding everything together. The base is going to have the rear corners tapered down (think Best Buy logo). The motors will be mounted to the bottom of the plywood base. Plywood sides will be used that will protrude above and below the base, covering the motors and holding two bearings to support the axles for the wheels (not sure what I'm going to use for axles yet). Above the motors, on the top of the base, will be two H-bridge PWM drives that I plan to build with ATTiny2313 microcontrollers. I also want to come up with some sort of encoder, probably a simple optical sensor, maybe paint alternating white/black stripes around the drill chuck and read it with an IR or visible light sensor.
I also want to add a mounting bracket of some sort to the front of the robot to attach accessories (arms, tools, vaccuum hoses, weed whackers, etc). This would also likely include a port for power and data to control the attached accessories.