The iconic mass-produced lager in Queensland, Australia, is XXXX (pronounced FourEx). On their label, they tell a story about how historically beers were marked with a number of Xs to indicate their quality, with three Xs being the best.
According to XXXX, the original brewer found that his beer was so good that he added a fourth X.
So what's the real story with the 'X' mark? I know there are other beers, such as Dos Equis, which incorporate an X in their name and branding. I have also heard of a number of Xs being used with whisky as a crude ABV indicator.