Modifying Servos for Continuous Rotation @ The Adafruit Learning System. Make tiny gear-motors for your next robot! Rick writes –

If you’re familiar with some of the other tutorials out on the Interwebs to modify servos for continuous rotation… you might be wondering what this guide has to offer.

Lots!

No Drift
Many of the mods / hacks I’ve seen use low precision resistors which will require you to poke around until you find what the servo thinks is center-position. That approach will require you to capture that value and use it as an offset in your software… which makes re-using your servos much more difficult. Using high-precision resistors is the way to go.

Grinders Away!
Cracking off a piece of the potentiometer housing is a quick and easy way to remove the servos rotational limits. It’s also a great way to make the servo wobbly and grind it’s gears; there is a better way.

Variable Speed
Matched resistors give you sweet and smooth variable speed coming or going.

I guess you could just buy continuous rotation servos – but why buy when you can make!