Looking for a good continuous-rotation servo? The Grand-Wing Servo GSV0008A is a well proven unit!
What makes "continuous rotation servos" so cool is that all you had to do is plug it into a regular style "Power/Ground/Signal" style RC receiver (or microcontroller) jack, and feed it a 1.5mS signal.
1.5mS means it stayed still. Go down towards 1.0mS, and it would turn one way, faster as you moved further away from the 1.5mS "neutral" signal. Then by changing the signal up to 2.0mS, you could make the servo spin the other way. This was very cool - you had a motor speed controller and gearbox all rolled into one tidy package.
Unfortunately, modifying a servo to do this meant you had to take it apart and perform mechanical and electronic surgery. Well, they clever people at Grand-Wing Servo have introduced their own standard, "modified-for-continuous-rotation" hobby servo!
This unit has all the characteristics of a regular servo, but it doesn't have a rotation stop. It'll keep on spinning. Slap a servo-wheel on it, and you've got a really simple way to make a mobile robot chassis!
And the price is excellent too!