A simple part that is like a rotator, but instead of being a position-control servo, it acts like the motors on the rover wheels. This would be excellent for building advanced contraptions like contra-rotating station arms, gratuitous centrifuges, and maybe propellers (100% not the reason I'm suggesting this). In order to facilitate faster spinniness, it would be awesome if a little maxAngularVelocity option was added to the Body XML element. Wheeeee! Not at all relevant to this or this.
Little edit: wouldn't it be cool if, when you spin it manually, it turns into a dynamo which puts energy back into the batteries? It's nowhere near an essential feature, and it might get a bit complex determining the load on the generator - this could probably be configurable but would fall off completely if there is no more 'space' in the batteries left. The load (current) would determine how much resistance there is on the motor... I don't know much really but it would be cool.
@WNP78 Yeah