http://www.pedros.com/visewhip.html
Leonard Zinn of technical writing and bike manual fame designed it and Pedro's made it. It works exactly like a good quality Vise Grip tool with double layer jaws that fit over the cog and engage the teeth with riveted pins.
Pedro's say it is good for cogs 11-23t. In practice, I adjusted it for a 13 or 14t cog, tightened the knurled locknut and it is always ready to go. The jaws are too narrow for 1/8'' track cogs but as I don't have any I could care less. If you need a little more leverage a 25mm i.d. cheater pipe slips over the rounded handle with the adjuster knob. I use an old seat post.
It's a little pricey at the $53USD I paid for it at
http://www.universalcycles.com/
but having used it I wouldn't be without it now.
And on a different note, here is Trichosurus vulpecula, the Common Australian Brushtail Possum. His name comes from Greek and Latin and means "furry tailed" and "little fox". When I was walking back to the house at night after photographing the Vise Whip, I spied this fellow from a distance preceding me through the open back door. When surprised he decided that the DVD shelf looked like the way out.
These omnivorous little marsupials are one of several Australian species named for a resemblance to their North American Opossum cousins. The Common Brushtail adapts well to life in suburbia. They are inquisitive and fearless and will colonise house ceiling spaces or outbuildings for daytime sleeping quarters. The territorial call of the male possum sounds like the death-rattle of a flesh-eating zombie and can be disconcerting for anyone unfamiliar with it.