Another day, another step forward in online-advertising domination for Google (NASDAQ:GOOG).

The world's leading Web company took its popular marketing platform to the casual-gaming space last night, when it formally introduced AdSense for Games. Publishers of online games -- built on Adobe's (NASDAQ:ADBE) Flash -- can now insert AdSense ads into their games.

Developers can incorporate text-based, video, or display ads. Google serves up the targeted ads between breaks in the gameplay, and the game sites share in the ad revenue. Google is initially limiting its roster of publishers to those who draw at least 80% of their traffic from the United States and the United Kingdom -- and for games with at least 500,000 plays -- but it will surely expand its reach over time. That's just the Google Way. Just as webmasters, big and small, can count on the original contextual marketing program to populate their sites with "Ads by Google," it's time for Google to put on its game face.

It doesn't hurt that Microsoft (NASDAQ:MSFT) is a big player in console-based advertising, after its purchase of in-game ad network Massive two years ago. With the war between Google and Microsoft heating up, the battleground has moved on to new terrain, such as the mobile-phone market. Casual gaming makes perfect sense, and it's also a booming industry. Google cites comScore data claiming that 25% of the world's Internet users -- or 200 million people -- play online games every week. If that's where the Web traffic is going, Google has no choice but to follow.

Casual gaming isn't an easy niche to break into, despite the simplicity of Flash-based games. THQ (NASDAQ:THQI) launched SlingDot two years ago -- in its stab at taking on Electronic Arts' (NASDAQ:ERTS) popular Pogo.com -- and the site is dead today.

However, one doesn't need a dedicated gaming portal to propel an online game into fandom. Between social-networking widgets and the wide variety of games selling briskly through Apple's (NASDAQ:AAPL) App Store, brief games that are easy to learn are popping up everywhere.

Google may not show up fashionably early to a hot trend soiree, but when it does, it's usually dressed to kill.

Step back, everyone. Microsoft and Google are going at it again.

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Microsoft is a Motley Fool Inside Value pick. Google is a Motley Fool Rule Breakers recommendation. Apple and EA are Motley Fool Stock Advisor recommendations. Try any of our Foolish newsletter services free for 30 days.

Longtime Fool contributor Rick Munarriz is always up for a good casual game. He owns no shares in any of the stocks 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 has a disclosure policy.