Romulan ale is the blue beverage that won't fill you up and never lets you down. It first appeared in TOS: "That Darn Khan!". It's true. You can look it up, those tight-asses at Memory Alpha notwithstanding.
It is widely known Romulan ale is illegal in Federation space, however, this rule is the only one ignored more than the entire Prime Directive since almost everyone owns, or has owned this beverage at some point in their lives. It is interesting to note that every Federation ship carries a supply for "diplomatic" or "medicinal" purposes and no one has yet been arrested for possessing Romulan ale in the entire history of the Federation.