I have designed and built a gripper for the Bioloid humanoid robot kit. The frame parts of the gripper are "printed" from a CAD model using an ABS 3D printer. It uses a GM-14a motor for wrist rotation, and a Firgelli linear actuator for the gripper. Control is via a custom ATmega168-based PCB. The gripper , when plugged in to the Bioloid, becomes just another device on the serial bus, and can be controlled in the same manner as a regular servo.
Some blog entries:
And finally, a picture of the robot, with two grippers installed: