This N20-based gearmotor  has 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, 3mmØ cylindrical output shaft, pre-ground with a flat spot for secure grub-screw mounting.

Here are the performance measurements:

Running at 3V:

• Free running RPM – 181
• Free running Current – 28mA
• Stall Torque – 5.9 in*oz
• Stall Current – 428mA

Running at 6V:

• Free running RPM – 370
• Free running Current – 42mA
• Stall Torque – 6.1 in*oz
• Stall Current – 760mA