20mm Gearmotor!
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These motors are beefy spur gearboxes, which means that it's a relatively simple compound gear train. spur gearboxes are generally not recommended in high impact applications (RE: combat robotics), but these have held up really well for me in the two events and sixteen fights I've run them in, only failing in a couple extremely intense matches.
These are offered by Pololu.com in 6V and 12V variants, and by Servocity.com in 12V variants. The 12V variants have seemed to have some issues when run on the Endbots_DESC, with some stuttering, so maybe stay away from that combination. I ended up just swapping the gearboxes off the 12V motors and onto some good motors with broken gearboxes.
These tend to weigh somewhere around 40g each, so they are not cheap in the weight department, but are one of the most powerful gearboxes you can reasonably get for Antweight Combat Robots.