In film and video, if you want to rack focus, a "follow focus" makes the job a lot easier. Attaching to the focus ring, a follow focus can be as simple as a stick or as complicated as a gearbox.
Ultimately, the purpose is always the same: Make it as quick and simple as possible to rotate that focus ring from one spot to another. And make the job one-handed.
But while follow focus is basically a standard in motion film and video cameras, it's nonexistent in the modern day, video-wielding dSLR world. So one flickr member made his own follow focus with a $6 steel hose clamp from Home Depot. Padded with a rubber band, it clamps right around the focus ring. And hey, it works great!
Look for the official, $200 versions from Nikon and Canon some time in the future, I'm sure. Unlike the Home Depot version, they will be powder coated black. [flickr]