A Round of Drinks for the House

In a moment of celebration (or madness), someone announces that everyone's drinks are on them! They might be a bartender, or a patron, or just an excitable party guest, but who cares who's paying for it? It's free booze!

Reasons to do this vary almost as much as reasons to drink in the first place. Maybe the buyer just won the lottery. Maybe there's a tense situation between a couple of patrons, and someone wants to avert a Bar Brawl. Maybe they're just drunk, stupid, in possession of a credit card and overcome with love for mankind. It's also a tradition for golfers who've scored a hole-in-one (be thankful -- in Japan, the tradition is to buy everyone expensive gifts), and for new fathers (known as "wetting the baby's head").

Frequently parodied when after a moment's thought, the announcer realises just how many people there are, and how much this is going to cost them...

Comics

 * This starts off the events of "PSmith" -- Al the bartender meets the Thuxian of his dreams, and it's free drinks for everyone.
 * Happens several times in the Lucky Luke comics. For example, when the Dalton Brothers come to a city in Canada, a gold digger arrives and uses his gold to buy a round. The saloon owner says that the gold diggers all do that and then go back to digging gold for another six months. Cue to Joe Dalton planning to take over the saloon...

Film
"If it should chance to be, we should see some harder days, Empty larder days, Why grouse? Always a chance we'll meet somebody to foot the bill Then the drinks are on the house!"
 * In The Muppet Movie, Fozzie defuses a Bar Brawl by impersonating the bartender and shouting "Drinks are on the house!" Of course, this being the Muppets, this causes all of the brawlers to race to the roof of the bar where they look futilely for the drinks.
 * Henry Chinaski's "For all my friends!" in Barfly.
 * Happens several times in The Quiet Man, and during the big fight scene, the bartender, Thornton and Danaher verbally fight over who's buying the drinks.
 * Red Tails has our heroes given a round on the house in the officer's club in thanks for their bravery.
 * Oliver! -- discussed (optimistically) in "Consider Yourself";


 * Paul Maclean does this in the gambling den in A River Runs Through It, believing he's about to have a lucky night.
 * The theatre producer does this in the inn in Shakespeare in Love, before exclaiming, apropos of nothing, "Oh, happy hour!"
 * In Avatar, after, Quaritch proclaims that the first round's on him tonight to celebrate.

Literature

 * Vagabonds of Gor: Tarl is in a makeshift tavern in an enemy encampment when a Boisterous Bruiser with whom he'd had previous dealings (to said bruiser's detriment) comes in to buy a round of drinks for everyone.
 * In Reserved for the Cat, Jonathan pretends to be someone who just won a lot of money and is buying rounds to celebrate as a way to keep a reporter occupied while Thomas and Wolf search the reporter's apartment.
 * In the Mercedes Lackey novel Phoenix and Ashes, Reginald routinely offers to buy rounds at the local pub. He's loaded and knows that the guys he befriended there could use a drink since many of them (like him) were sent home after being injured in WWI.
 * In Snuff, Vimes several times buys a round for all the patrons in a bar situated on his land, generally as a way to earn their gratitude, and once in an attempt to avert a Bar Brawl.

Live Action TV

 * My Name Is Earl: Earl & Randy discovered that if a golfer hits a hole in one the tradition is for that golfer to buy a round for everyone in the clubhouse. So they make sure that that golfer gets a hole in one every time.
 * Subverted in the short lived detective series Mayo, where the titular character would say this to his team and be turned down by all of them at the end of every episode.
 * Buffy the Vampire Slayer. Warren announces that drinks are on him at the local demon bar to celebrate the fact that he's a Big Bad who killed the Slayer. The demons respond by laughing at him, because they've already heard that Buffy survived the attack.
 * How I Met Your Mother has Barney loudly declare he's buying champagne for everyone in the bar. After the cheers die down, he quietly tells the bartender to give everyone ginger ale.

Music
"When, out of the night, which was dark and cold, into the smoke-filled dimly lit room Stumbled into thug who smelled like bud, and his eyes looked high as the moon He looked like a man with his foot in the grave, and his lifetime 'bout to be out Yet he slapped down some hundred dollar bills on the bar, and he yelled out "Drinks on the house""
 * Big and Rich's song "Save a Horse, Ride a Cowboy" has this line- "And I buy the bar a double round of Crown".
 * In Jason Michael Carrol's song Where I'm From, the narrator's in the first class section of a plane. The man next to him says that the drinks up there are free, and he says "the first one can be on me".
 * The song "Solomon Jones" by Aceyalone & RJD2.

Tabletop RPG

 * Dungeons and Dragons. In the 1st Edition Dungeons Master's Guide, buying a round of drinks for the house was part of the cost for "Frequenting Inns and Taverns", one of the techniques for finding potential henchmen.

Truth in Television

 * In 2004 a businessman at a London nightclub said "The drinks are on me". The nightclub patrons ordered 110 bottles of champagne, and he ended up with a bill for more than 42,000 pounds ($59,000).

Western Animation

 * Moe, on The Simpsons, celebrates Homer and Marge reaffirming their love by announcing "Hey, everybody! For the next 15 minutes, 1/3 off on every pitcher! ...only one per customer... Hey, no sharing!"
 * In Rugrats, "the drinks are on me" resulted in everyone dumping their drinks on the character in question's head.
 * Parodied in Tex Avery's The Shooting Of Dan Mcgoo, when someone calls for drinks on the house. Sure enough everybody runs to the roof and starts drinking.

Video Games

 * In Mass Effect 2, Shepherd can buy drinks for all of Afterlife's patrons. The bartender tells him/her that there are a lot of patrons there that night and gives them the option of weaselling out.
 * This is a gameplay mechanic in the Uncharted Waters series: ordering a round of drinks for the entire tavern gives a temporary bonus to how many sailors you can hire there afterwards. The more money you spend on the drinks, the larger the bonus.
 * In Mount & Blade buying booze for everyone in town increases your reputation for that town.