Chris Holloway’s Beer Gauge, which measures the amount of beer is in a pint glass:
The problem is the design of the pint glass. From a serving point of view, it’s hard to fill a pint glass to the top. There’s all the sloshing around and so forth. But some bartenders really don’t understand a pint is a true measure. I had a bartender tell me about six months ago that my Beer Gauge would not work in his bar because his pint glasses range anywhere from 14 to 20 ounces. So I said, “Well, it’s not a pint glass then, is it?”
Semi-related is the Honest Pint Project, a directory of pubs that actually verify they pour 16 ounces of beer.