Dominant companies always look invulnerable–until they’re very vulnerable. That’s been the case throughout the modern history of technology, with IBM, Microsoft, Yahoo, and MySpace all once upon a time seeming secure at the top of the mountain. And then, one by one, each was toppled (though Microsoft is still a $638 billion company).

Such is the case today, as four companies–Google, Apple, Facebook, and Amazon–vie for tech’s Iron Throne (with a combined market cap of $2.61 trillion). But while there are no credible contenders for the crown today, who knows what will happen a few years down the line.

The key to successful revolutions is fundamental innovation, and in the case of the tech industry, there’s ample evidence we’re on the cusp of several new categories that could produce giant, multi-hundred-billion-dollar winners. It’s possible, of course, that Google, Apple, Facebook, or Amazon could be the companies to best capitalize on those new innovations, but then again, it could be altogether new players.

Even if newcomers don’t become giants, there’s still room for big new successes. “It will be very tough to create a general-purpose search engine, or a mass-market social network now,” says Benedict Evans, a partner and analyst at Andreessen Horowitz, one of Silicon Valley’s most prestigious and powerful venture capital firms. “It would be very tough to create an Apple or Facebook, but that doesn’t mean you can’t compete with the things they’re doing.”

The question then, is what are the technologies that are going to drive the future. At Andreessen Horowitz, a…

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