Here are two Hextronik HXT900 microservos which I have modified for continuous rotation. These will provide the propulsion for the latest version of Grisbot.
HXT900s are about the cheapest microservos out there. I’m planning to buy mine at Hobby King for $2.69 each, which is much nicer than the $15 or so that the bigger servos cost.
It took me about half an hour to convert the latest servo, which is way too long. However, I’m getting the hang of it. I think that with a little practice, I can get it down to no more than five minutes per servo.