Romulan ale was a strong, blue-colored alcoholic beverage created by Romulans. In the 23rd century it was illegal to posess in the Federation, but some low-level smuggling took place nonetheless. (TOS movies: Star Trek II: The Wrath of Khan, Star Trek VI: The Undiscovered Country)
In approximately 2258 of the Nero created alternate reality, Quocch became intoxicated on 14 Romulan ales (his payment for a completed smuggling job). He claimed he would purchase his own planet while drunk. (TOS comic: "Vulcan's Vengeance, Part 1")
In early 2366, Quark's bar employee, Rom, accidentally tipped Romulan ale over a Cardassian patron, which led to all the occupants of the bar pouring drinks over Rom's head. (TNG - Double Helix novel: Vectors)
- See also: kali-fal.