Here's a little video of my work-in-progress robot rover. It uses a Raspberry Pi, running OpenCV with a webcam, to see and react to its environment.
The current program is just a proof-of-concept prototype - it looks in the field of view for any strongly green objects. If it can see one, it turns to face it, then advances until it is close. If it can't see any, then it turns on the spot until it can.
The power supply turned out to be one of the more difficult aspects, and there's still some way to go. Initially, I had the motors running from the Pi's 5V rail, decoupled with a fairly large (470uF) capacitor. But it seems that the H-Bridge can supply quite a lot of current when braking the motor, and every time I tried to change the motor direction, the Pi rebooted!
Now, I have the motors supplied from a separate 4.5V AA battery pack, which seems to work fine. Providing battery power to the Pi is troublesome, since it can draw a lot of current. For now, I'm running it with an umbilical power cord, but I plan to try one of those cheap 18650 battery boxes, that use high-capacity cells running through a DC-DC boost converter to achieve a stable 5V at up to 2A.