Last month I got the chance to go to Bangkok to attend a salesianity seminar. Having one day before my flight, I decided to look for this company where they sell inexpensive simple programmable robots.
So there's this trio for like $25-$35 each (more expensive at e-bay). The blue one with two sensors is a line tracer, you have to bend the sensors downwards towards the floor for it to detect your black line and follow it. The middle one is a sumo robot and will stay in a circle of black and try to push away all its foes unless it gets pushed out first. And the red one is an obstacle avoidance robot which goes around and will try not to hit anything or anyone in its way. I forgot to include in the picture the light follower.
They have the six pin ISP socket for advanced programming of their Atmel ATmega168.
They can be found in the Future Kit website.
http://www.futurekit.com/2009/index.php?option=com_content&task=view&id=1263&Itemid=350