If you're looking for a speedy motor with a reasonable amount of torque (for say, moving a small robot that isn't going to fight in a Sumo contest), the GM11a is an excellent candidate. This particular motor has a 9.4mm long, 3mm dia. cylindrical output shaft, pre-ground with a flat spot for secure grub-screw mounting.

Here are the performance measurements:

Running at 3V:

Running at 6V:

• [Data Sheet]: Solarbotics Gearmotors Specification Summary
• [Data Sheet]: GM11a Performance Datasheet PDF (155KB)
• [Data Sheet]: GM11a Performance Datasheet PDF (155KB)