Circuit Cellar magazine recently posted in full their two-part article by Tom Kibalo on the construction of his subsumption-based mobile robot, called TOMBOT. Part 1 of the article covers construction of the hardware and Part 2 covers the subsumption software and basic behaviors for obstacle avoidance, collisions, and light tracking. The robot is a differential drive design using continuous-turn RC Servos as motor. It lacks wheel encoders. The robot sports an XBee radio, a PIC32 CPU, and a small LCD display. It's a good basic introduction to behavior-based robots and well worth a read.