It's a nub brazed onto the right seatstay just above the dropout. The chain can be hung from it when the rear wheel is off. This keeps the chain under tension and more under control if you transport your bike with the wheel off.
Here are a couple pictures from one of my bikes. The second picture shows the chain on the chainhanger (though it isn't normally used with the rear wheel on).
Last edited by DebW; 10-19-2008 at 06:33 PM.
Oil is good, grease is better.
2007 Peter Mooney w/S&S couplers/Terry Butterfly
1993 Bridgestone MB-3/Avocet O2 Air 40W
1980 Columbus Frame with 1970 Campy parts
1954 Raleigh 3-speed/Brooks B72