A bit of royalty has joined the family tree at RealNetworks' (NASDAQ:RNWK) collection of casual gaming sites. The online multimedia pioneer inked a cross-promotional deal with King.com, making King the exclusive skill-game provider on RealNetworks' RealArcade, Zylom, and GameHouse sites. In return, Real will offer its casual and downloadable games through King's site.

It's OK if you don't associate RealNetworks with games. The company's RealPlayer media software and Rhapsody music subscription service are the company's two most notable offerings. However, RealNetworks' casual gaming empire also encompasses a community of 40 million unique monthly players. Even if we concede that those folks are mostly penny-pinchers looking to play free games, it's still a hefty slice of the online population, and an attractive captive audience for marketers.

Casual games are big business. Electronic Arts (NASDAQ:ERTS) acquired Pogo in 2001, but things quieted down shortly afterward, when the dot-com bubble popped. Things have starting picking up lately, though. Earlier this month, Viacom's (NYSE:VIA) Nickelodeon committed to spending $100 million in casual games.

Don't let the kid-friendly Nick crowd, or the diehard teen gamers who worship EA, fool you. Casual gaming is still a unique niche, in which most players are adult females. Brief, mentally engaging games have proven appealing to stay-at-home moms, retirees, and even bored office workers.

The deal with King makes sense for RealNetworks, especially since it's able to cash in on many licensed TV-related King games based on hot properties such as American Idol, Deal or No Deal, and 1 vs. 100.

The King partnership comes just days after RealNetworks announced that it would team up with Mattel (NYSE:MAT) to distribute downloadable versions of Mattel board games such as Scrabble and Uno.

So don't be surprised if the next time you hear the name "RealNetworks," you begin thinking about virtual letter tiles, or taking on a caustic Simon Cowell or Howie Mandel's banker in cyberspace. With 40 million gamers, there's certainly room for one more.

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Longtime Fool contributor Rick Munarriz has been known to break into an online game or two. He does not own shares in any of the companies in this story. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool's disclosure policy doesn't play around.