Over at Trossen Robotics Forums, I became aware of a sweet USB turret with the ability to track multiple targets. They built it using Phidgets (they are on our holiday gift guide) and a few other parts.
In addition to just servos, I found out via Trossen Robotics you can use a servo controller hooked into a motor controller to control it since not all motor controllers have a USB or serial port on it. Nice bridgeware.
Also if you haven't heard about Trossen Robotic's contest just for building cool stuff, check it out.