These N20 Gearmotors are perfect for fairyweight and antweight robots.  These are the motors we use in all of our antweight robots including 2 time CIRC champion "Death by 1001 Cuts"  (it used 30:1 motors)

The motors are fragile compared to other offerings such as the Fingertech Silver Sparks, but they come at a significantly lower cost.

Specifications

Weight: ~10.0g