My son William and I just finished assembling a new robot brain. It is the Robot Builder's R-Duino (RBR) from Ro-Bot-X Designs.
Like all of Ro-Bot-X's work, the board is pretty well thought out, and simply packed with features and options.
The major advantage of this board is that it includes an Arduino processor, connections for sensors and servos, and drivers for two motors all on one Arduino form factor board. That means you can add standard Arduino compatible shields to the R-Duino and get new features easily.
I decided I wanted a 4WD robot, so I bought a motor shield from NKC Electronics. This shield uses the same SN754410 motor driver chip as the R-Duino, but it uses different digital out pins to control it. Combined with the R-Duino, it should give me independant control over four motors.
While I'm waiting for motors to arrive, I popped the R-Duino on an robot chassis I have lying around, Mr Mountain Monster (MMM).
The motors are really too much for this board, so I'm running it at about half speed using Pulse Width Modulation (PWM). It's all just temporary. I'll pull the R-Duino off MMM when my new parts get hear and I can test a 4WD chassis.