I've done this project based on the line follower in the book Arduino Robotics by Warren, Adams, and Molle.
A while ago, I got hold of a car alarm unit and its two remote control fobs. When i hooked it up to 12V I realized that it didn’t work, not responding to any of the commands from the two fobs even though their batteries were OK. Naturally, I opened up the unit to start harvesting parts and noticed the RF module within (the one that sticks out perpendicularly) and I figured it shouldn’t be too hard to use the module with an Arduino.
I am helping my friends first robot! He primaily wanted tractor treads and an Arduino "because he heard they were good" ( I didn't disagree with him. :-)
We decided on an inexpensive kit was a good starting point and then it can be expanded later.
Our choice was the BrutusBot and electronics package (looks like I get to solder 2 boards, lucky me...). It was fairly cheap.
He seems good in the programming and mechanical areas, he isn't so good with the electronics part (he does fine with some guidance).
Well, finally I've started to assemble my first go at building an autonomous bot, nothing fancy, nothing original just something to learn the ropes. I'm going for a 2WD design that to me seems to be quite ubiquitous.
As of the moment (May, 17th 2012) basically I've only glued together a couple of DVDs (following generalgeek's sugestion) to use as base, and then also hot-glued a couple of DC motors and a 3rd non-motor wheel to it.
Component-wise I'm only waiting for a couple of transistors to build an H-bridge (as sugested on a Dan M's post).
My robot is a hacked RC car controled by an Arduino Duemilanove. It has no sensors, but I have left room on my chasis so I can add some more later when I feel more comfortable around Arduino. Most of the time on this project was spent dealing with Arduino code and the motor shield.