Here’s a guide on modifying extremely cheap servos to rotate continuously. These servos can be used as drive motors for small robots etc. The guide can also be used for different types of servos too – Link.
Editor at large – Make magazine. Creative director – Adafruit Industries, contributing editor – Popular Science. Previously: Founded – Hack-a-Day, how-to editor – Engadget, Director of product development – Fallon Worldwide, Technology Director – Braincraft.