In fairy tales, for every giant, there's usually a giant-killer. It might take a while for an individual to emerge who has the savvy, nimbleness, and ability to take down a giant, but it always happens -- usually once the giant gets bigger, slower, and more unwieldy. Of course, business isn't about fairy tales, but some companies take on an almost storybook star power as they shoot their way to the top.

An example of a corporate giant-killer might be Netflix's (NASDAQ:NFLX) Jack to Blockbuster's (NYSE:BBI) giant. In its day, Blockbuster capitalized off film's transition to a rentable form. Its blue and yellow logo became emblematic of movie rentals, but it eventually became big enough and complacent enough to miss the wildfire spreading of Netflix's grassroots appeal.

What about some of the other companies that are, or once were, Rule Breakers but have now reached giant status? Are there threats brewing? In true Rule Breaker fashion, it seems that some of them come from the oddest places.

Giant No. 1: Whole Foods Market
Ah, Whole Foods Market (NASDAQ:WFMI). Just when you think there are no innovations left in a certain industry -- and let's face it, for a long time the grocery industry was very boring -- somebody comes along and revolutionizes it. Whole Foods Market defied conventional wisdom by opening an entire grocery chain that provides nothing but organic foods and other natural fare.

When Whole Foods started out, skeptics believed that there wasn't a sustainable market for such fare. They believed that when regular grocery stores started offering organic items, Whole Foods would be put out of business. Instead, Whole Foods prospered; CEO John Mackey has said in the past that when regular grocery stores offer a limited number of organic and healthy alternatives, it merely exposes consumers to the products and drives many of them onward to the full Whole Foods experience.

For the time being, Whole Foods is the unmistakable winner in this area. Does anyone else compare? Well, Whole Foods runs circles around one major competitor that springs to mind, Wild Oats Markets.

If I were to think of a stealth rival that might pose a risk, though, I'd surmise that maybe, just maybe, it's eclectic, funky little Trader Joe's. A lot of people like to joke that Whole Foods should be called "Whole Paycheck" because of the high prices of its goods. That's where Trader Joe's goes for the jugular. Not only does it provide a combination of gourmet, organic, and healthy fare, but its products are also cheap. TJ's claim to fame is cutting out the middleman and paying suppliers in cash in order to pass along the savings to customers, who I would argue include the same demographic that Whole Foods targets.

Trader Joe's already has more stores in the U.S. than Whole Foods has, and it aims for continued expansion into new markets. Look out.

Giant No. 2: Google
Once upon a time, a search engine with an odd name seemed to come out of nowhere. Suddenly, the word "Google" became part of common speech. It's a giant now with a capital G. At my last check, Google (NASDAQ:GOOG) shares were trading at about $300 a pop, giving it a market cap of a whopping $84 billion. At the moment, most people view it as a clear Internet winner, expected to win in just about every area it touches.

Is Google invincible? Of course not -- no company is. However, given Google's search expertise, its killers may not be Internet heavyweights like Yahoo! (NASDAQ:YHOO) or Microsoft (NASDAQ:MSFT). I had a recent chat with David Gardner, and he theorized about a candidate that could possibly cripple Google in search, where it is strongest: Wikipedia -- the information clearinghouse run by the nonprofit Wikimedia Foundation.

Quietly, stealthily, private Wikipedia has amassed a wealth of information through the power of community, becoming an "open source" answer to traditional reference material. People pull from their own expertise to update Wikipedia's entries, making it the major Internet encyclopedia -- and it's free of charge. It recently became the most popular reference site, with traffic up 154% in one year, according to Internet monitoring firm Hitwise.

Indeed, when you're looking for a specific answer to a specific question, a definition of a term, or such, Google searches can at times feel like looking for a needle in a haystack, whereas Wikipedia doles out instant answers. Ouch.

Giant No. 3: eBay
 eBay (NASDAQ:EBAY) was a big Rule Breaker in its day, revolutionizing e-commerce by forming a virtual bazaar. It had a grassroots appeal that quickly caught on with thousands of people before the company even had any need to execute a marketing campaign. However, as the years have passed, eBay has become more and more of a behemoth, spawning competition from many Internet heavyweights that coveted even just a share of eBay's success.

However, it seems to me that its biggest competition has come from one of the most unlikely of sources. Consider Craigslist, a remarkably anti-corporate operation that sprang out of nowhere and connected people to buy and sell goods, meet one another, find jobs, discover cultural events, and much more. Combining the concepts behind consumer-to-consumer auction with a touch of social networking, Craigslist is one of the most popular sites on the Internet.

Most impressive of all, Craigslist is free for users (it does charge for employment ads in San Francisco, Los Angeles, and New York), and it's devoid of the advertising that is so pervasive on the Internet. These are elements that are probably refreshing for the people it attracts.

Again, this is a private concern with no known plans to go public; regardless, its power has become obvious. Apparently, eBay has seen the writing on the wall when it comes to this scrappy little upstart, seeing how it has dabbled in real estate classified advertising by snapping up Rent.com. And, in a development that underlined the idea that Craigslist is a real contender, eBay picked up a 25% stake in the upstart at its first opportunity.

The David and Goliath effect
When it comes to these Goliaths and the Davids who face them, I have noticed an interesting connection. The upstarts I have mentioned have some fairly surprising qualities. For example, they're all private concerns that haven't really paid to evangelize their products and services -- their success has mostly occurred through viral word of mouth.

Many of these entities seem to reject what some might consider traditional corporate values by operating through more grassroots methods with an emphasis on community. In this day and age, when many sophisticated consumers are becoming more jaded about corporations, intrusive marketing, and a slew of other concerns, I argue that the rejection of tradition and convention make these upstarts formidable rivals.

Perhaps these small companies ultimately don't stand a chance against the giants they've taken on. But then again, maybe they've got just enough juice -- and individualism and creativity -- to topple the past innovators. If there's anything a Rule Breaking investor should keep an eye on, it's the new breed of Rule Breakers that are developing behind the scenes, quite possibly in the most unexpected of places. To keep abreast of the issues, talk about the possible rivals, and uncover new high-growth investments, take a free 30-day trial to Motley Fool Rule Breakers today. You'll immediately enjoy access to one year's worth of recommendations, research, and buy reports, as well as the recently released annual review, in which the Rule Breakers team identified the one stock most worthy of new money now.

Alyce Lomax does not own shares of any of the companies mentioned. Whole Foods, eBay, and Netflix are Motley Fool Stock Advisor recommendations. The Motley Fool has a disclosure policy.