Servos are among the most useful components in the inventor's arsenal. I know that whenever I need to make something move on it's own, a servo is usually the first thing I reach for. A couple years ago, CCCKC member Craig Berschiedt taught me how to turn a normal RC project servo into a continuous rotation servo. The trick involves modifying the mechanics of the axel to rotate a full 360 degrees, but also to modify the electronics to allow for controlling speed and direction of rotation rather than position. Basically, you replace the existing potentiometer with a pair of resistors for a hard-wired voltage divider, and the remaining circuitry takes care of the rest.

Now he has taken the hack one step further, and replaced the mechanics of the servo themselves, leaving a relatively cheap DC brushed motor controller. More details can be found on his blog or on thingiverse. Nice work, Craig!