Beer is the only consumable I can think of that is so associated with its standard serving measure that you can say, "let's go for a pint," and everyone knows what you're talking about—it's not a pint of milk.
But is a pint always a pint? It depends on where you are. Some countries legislate the size of a beer, but in the United States alcohol laws tend to be aimed solely at restricting where and when it's sold. A standard U.S. pint is 16 ounces (473 milliliters), but it's not governed by law—bars can serve beer in whatever size glasses they want.
Things get more confusing overseas, as I learned on my recent visit to Australia. There, beer sizes vary from state to state. In Sydney, most people order by the schooner—or, because Australians give everything a nickname, the "schooey"—which is about 425 milliliters. In Melbourne, you can order a pot, which is 285 milliliters, according to this handy-dandy chart. Many pubs (frequently called, confusingly, hotels) also offer pints, but in my experience Australians preferred to order the smaller sizes. This is not necessarily a reflection of a culture of moderation (also in my experience), although drunk driving laws are very strict there. My guess is that because it gets so hot there, shorter glasses make it easier to keep a cold one always at hand. On one very muggy afternoon walking around Melbourne, it was great to be able to duck into a pub for a quick pot whenever we needed a respite, without getting soused.
Not so in the mother country. In Britain, the Imperial pint—equal to 568 milliliters—has been the legal beer measure since 1698, and woe to the publican who pulls a short pint. On request, half- or third-pints (for a sampler) are also available. Now the country is considering amending the law to allow a new size akin to the Australian schooner, or about 3/4 of a pint. The concern isn't less-than-frosty beer—Brits have a not-entirely-accurate reputation for preferring their brews warmish—but public health. As the Associated Press reported, while per capita alcohol consumption had decreased in many European countries since 1970, in Britain consumption had increased by 40 percent. The hope is that, given a choice of a smaller serving, many people will drink less at a sitting.
Germany, a country so serious about its brew that its Reinheitsgebot dictated what ingredients could be used in German beer, keeps it simple with the metric system. There, the Maß—a full liter, or nearly double an Imperial pint—is the standard glass size, though half-liters are also available. I'm guessing this was not one of the countries where beer consumption has declined. Of course, with those heavy beer steins, you could probably consider drinking beer in Germany a weight-bearing exercise. Watch out Zumba—I smell a new fitness craze on the horizon.