Royalties Heir

Someone, usually a supporting main character but not always, is obscenely rich. How did they get that way? Simply put, because their mother's uncle-in-law invented hole punchers. Their great-grandfather invented spark plugs. Their grandmother wrote one of the most popular novels of all time. Their father invented Toaster Strudel.

Basically, their (insert relative here) invented or created (insert amazingly widespread, popular, or useful object here).

Not to be confused with the heir of royalty.

Examples of Royalties Heir include:

Film

The Toaster Strudel example mentioned in the description is from Mean Girls.
In About a Boy, Hugh Grant's character is obscenely rich and layabout because his father wrote a ridiculously popular Christmas song.
An early draft of The Big Lebowski explains that the only reason The Dude has any money is because he's related to Erno Rubik.

Literature

In the Nancy Drew graphic novel "The Fake Heir", the millionaires central to the action made their fortune designing fax paper.
Lily Rowan in the Nero Wolfe stories is rich because her father built the New York City sewer system.
From Larry Niven's Known Space stories, professional thrillseeker Gregory "Elephant" Pelton is a billionaire because his grandmother invented the transfer booth.
Subverted with the protagonist of the Kiki Strike novels: her great grandfather invented control-top panty hose, but her family can't spend it because it is locked away in a trust fund that can only be spent on education fees.
In Good Omens, one of Anathema Device's ancestor invented... the device. (Which is a part of clocks.)

Live Action TV

Roxy from Dead Like Me invented leg warmers. Unfortunately, her jealous roommate killed her and stole the idea, so she never got a chance to get rich or to have kids to pass the wealth on to. The murderous roommate, on the other hand, is now living the high life.

Theater

Mentioned in "Give Them What They Want" from Dirty Rotten Scoundrels The Musical:

"Her family fortune is obscene. Her/Dad invented Orangina."

Western Animation

In Danny Phantom, Sam's grandfather invented the machine that twirls cellophane around deli toothpicks.
In The Looney Tunes Show, Bugs gets all his money from having invented the carrot peeler.

Real Life

Former Monkee Mike Nesmith's mother invented White-Out, making him very, very wealthy.
Taken advantage of by Lorraine Williams, the former CEO of TSR, inc., the company that published Dungeons and Dragons. As one of the company's side projects, she devoted massive time and resources towards creating and promoting a Buck Rogers RPG, including sourcebooks and computer games, despite the fact that the market for such a game was just on the plus side of nil. Naturally, the owners of the Buck Rogers trademarks would receive royalties for each such supplement sold. However, it just so happened that Williams herself was the heiress to those properties, which is just about as conflict-y as conflicts of interests get.