Pololu's Round Robot Chassis
This circular laser-cut plastic robot chassis makes a great base for small mobile robots. With a diameter of only five inches (slightly bigger than a CD), this acrylic chassis is ideal for building sturdy robots capable of navigating tight spaces. As shown below, the circular chassis has many holes and slots for mounting sensors, motors, and other hardware.

To complete the round robot chassis, the kit includes a Tamiya twin-motor gearbox, ball caster, and truck tires;