What is a beer warmer?

I have used one at a good bar in town run by a Czeck guy who's quite a beer afficionado. It is a stirling silver cylinder with a little hook on the end. The cylinder has a cap that you fill with hot water. You drop the cylinder in your too-cold beer and the hook holds to the edge of your glass so it doesn't slide all the way in. In a minute or two the temperature of the beer has fallen a few degrees. It's great for dark ales and other beers that you don't want to drink too cold but are kept in the same fridge as the rest.