What's black and white and read all over? The Internet has changed the punch line to that old riddle, and forget about newsprint on your hands. Further, one might argue that a small, private, no-frills operation called craigslist is changing even more of the rules.

A recent report from Classified Intelligence, a consulting firm, claims craigslist has sapped up to $65 million in classified advertising revenues -- specifically employment-related revenues -- from San Francisco-based newspapers alone. It stands to reason this is the tip of the iceberg.

Here at the Fool we tend to occupy ourselves with publicly traded companies, but we also ponder Rule Breakers. Craigslist may be private, but it's notable -- instead of an explosion in venture capital and in-your-face marketing, craigslist grew almost without notice.

These days, when it comes to buying, selling, renting, and trolling, it's hardly uncommon to hear someone say, "Go check craigslist." According to USA Today, craigslist's audience grew a staggering 800% from August 2001 to August 2004 -- with no marketing to speak of, save word of mouth.

Its impact isn't a concern only for newspaper companies such as Hearst Corp., New York Times (NYSE:NYT), and Tribune (NYSE:TRB), but also for other companies that offer classified advertising, such as Yahoo! (NASDAQ:YHOO).

Indeed, Internet-based job sites such as Monster.com (NASDAQ:MNST) could also suffer, and even online dating services. With people using craigslist as a way to meet others in their neighborhood (for free, too), it has an impact on those trying to profit off the idea, such as Yahoo!, InterActiveCorp's (NYSE:IAC) Match.com, and private sites like eHarmony.

Of course, you can sell your stuff on craigslist, too. Which brings us to the big heavyweight in community-based buying and selling, eBay (NASDAQ:EBAY). Both eBay and craigslist boast loyal communities, although craigslist's can be described as grassroots. Is eBay worried? Well, it did take a 25% stake in craigslist this past summer.

It's not like eBay hasn't been known to break a few rules itself. Its own recently announced purchase of Rent.com shows that it's already seeing the sensibility in craigslist's simple model, which also includes real estate listings.

What's the secret? Craigslist appeals to the young and hip. It's rebelliously anti-corporate; eBay purchased its stake from a former employee, and craigslist's official response was that it doesn't seek money from outsiders. The site lacks pop-ups and banner ads, and it has no graphics and hardly even any color on the site. It's not even a dot-com; it's a dot-org. Its browser icon is a peace symbol. It's perceived as cool, and most importantly of all, it's free.

The small crew that works for craigslist, which includes of course the list guru himself, Craig Newmark, doesn't seem too worried about making millions, though it appears the service is making millions happy. Those who might want to worry are some of the companies that thrive off communities and classifieds. It seems there may be a new star in your town, and his name is Craig.

Craigslist may have broken rules, but it's a private company. David Gardner's been picking out stocks that defy conventional wisdom and promise heated growth in Motley Fool Rule Breakers . To take a 30-day free trial, click here.

Alyce Lomax does not own shares of any of the companies mentioned.