Here is the robot platform I developed for use in robotis tutorials at The Sydney hackerspace (Robots and Dinosaurs)

 

I designed it to be cheap, but it still has some great features.

 

• The plastform ready to follow a line costs less than $20 USD.

• It has a rechargable, protected LIPO battery built in.

• The arduino is mounted in a breadboard on top, and everything interfaces through it, making it super easy to connect new hardware.

• All the 'runing gear' is mounted underneath the chassis, so the top is clear for further development.

• Everything was held on with hot glue or cable ties.