The robot on the top is controlled by the Power-Functions remote through the home-made IR link that the robot carries. The NXT reads the IR commands from the link and uses them to control the motors. Channel 1 on the remote controls the drive motors, and channel 4 controls the grabber. The program that runs on the IR link is the same program that I posted earlier, which is able to both send Power-Functions commands and to receive them. An NXC program on the NXT parses the 16-bit command to extract the channel and specific command (this was facilitated by Lego’s publication of the protocol).
The little vehicle on the lower left is a conventional Power-Functions vehicle, using a battery box, a receiver, and two medium motors (one for each large wheel). It receives on channel 2, so the same remote can control both it and the robot.

Click on the image to watch a video of the two in action.