Super Mario Bros. Plumbing

Mario Brothers Plumbing (sometimes shortened to Mario Bros. Plumbing) is the plumbing business owned by Mario and Luigi, which, despite never appearing in the games, was featured in various cartoons and the movie. It is located in Brooklyn, in the neighborhood of Flatbush.

In The Super Mario Bros. Super Show!, it is located on the basement floor of an old apartment building, Mario and Luigi have succeeded in not only using the place as a workshop, but as their place of residence as well. As the basement is not a proper apartment, the brothers have learned to adapt to their surroundings, including the building's sneezing furnace and the garbage chute. Due to the fact that the building is located next to the closed-off entrance of a subway station, anyone in the basement can feel the rumble of an incoming train.

The Super Mario Bros. Super Show!
Mario Brothers Plumbing was the setting of all the live action segments of the The Super Mario Bros. Super Show!. Here, Mario and Luigi interacted with their roommate Edison, as well as the resident monsters Clog, the Ratigator and the CooKoo Bird; and in each episode, a celebrity guest star.

As mentioned, the old apartment also has a furnace which seems to have a life of its own (it sneezes) and it also has a Pizza Phone (originally a normal phone but due to an accident, the pizza and the phone got mixed together), a periscope apparently able to see from Brooklyn even all the way to the Eiffel Tower in France, and a garden being grown in a bathtub with a living scarecrow.

The motto of the brother's plumbing in the show was "You clog 'em, we clear 'em" and they were said to be New York City's best plumbing service. A commercial bumper for "Neatness Counts" stated that their motto was, "The customer is always wet."

The Adventures of Super Mario Bros. 3
In The Adventures of Super Mario Bros. 3, Mario Brothers Plumbing only appears in the show's opening, where Mario and Luigi were watching a photography album of the events in Super Mario Bros. 3.

Nintendo Comics System
The Mario Brothers Plumbing made a small appearance in the Nintendo Comics System short-story, "The Legend"; oddly, it is never actually stated as to whether this version of Mario Brothers Plumbing is on Earth or the Mushroom World.

Super Mario World television series
The Mario Brothers Plumbing appeared once in the Super Mario World animated series, during a flashback in "Rock TV", where Mario and Luigi remembered an outlandish phone bill for the "Speak to Santa Hotline", which presumably lets users talk to Santa Claus.

Super Mario Bros. film
In the Super Mario Bros. movie, Mario and Luigi's business is in their home and they simply stay in their house until they get business calls. In this portrayal, they own a white van with "Mario Bros. Plumbing" written on the side of it.

Trivia

 * Oddly enough, in Super Mario Bros.: Peach-hime Kyushutsu Dai Sakusen!, the Mario Bros. are not plumbers, but instead own a grocery store.
 * Though their actual plumbing business is never seen in the games, the fact that Mario and Luigi are indeed plumbers has been mentioned consistently in the video game series, and they have actually been seen plumbing in games such as Mario Bros. and Mario & Luigi: Superstar Saga. Also, Super Smash Bros. Brawl claims they were plumbers in the description of the Shellcreeper's trophy.