OpenServo is an open source project with the goal to create a digital servo for robotics that is free for anyone to use and modify to meet their particular needs.
See http://www.openservo.com for more details.
During the time as a student I've participated at this great project. Unfortunately I hadn't the time lately. Anyway the guys did a good job and have developed the new version 3.0 with some really nifty features like speed sensing through measurement of the back-emf.