NEW YORK (Fortune) -- Blogging tends to be personal, social, lively and irreverent. Does that sound like a big corporation to you?

It doesn't to me either, but "corporate blogging" is no longer an oxymoron--or a rarity. According to a running tally of big-company blogs, more than three dozen of the FORTUNE 500 companies now produce blogs, for better or worse.

For better? Check out the writings of the world's best known CEO blogger, Jonathan Schwartz of Sun Microsystems (Charts). Sure, there's geek-speak. "Thumper (sorry, the x4500) is built atop a 2 socket Galaxy server, it leverages Solaris/ZFS...and has 24 terabytes of serial ATA disk inside," he wrote recently.

But Schwartz also offers straight talk: "Most people in the world will first experience the internet on their handset." And regular readers get a sense both of the issues facing Sun and the life of a tech company CEO. One of Schwartz's postings began like this: "I had lunch with Tony Blair today. (And yes, I have been waiting all afternoon to type that.)"

For worse? A blog praising Wal-Mart (Charts) called "Wal-Marting Across America," ostensibly created by a man and a woman traveling the country in an RV and staying in Wal-Mart parking lots, turned out to be underwritten by Working Families for Wal-Mart, a company-sponsored group organized by the Edelman public relations firm. Not cool.

This week, Richard Edelman, president and CEO of the firm, apologized on his own blog: "I want to acknowledge our error in failing to be transparent about the identity of the two bloggers from the outset. This is 100% our responsibility and our error; not the client's."

The lesson's clear. The best corporate blogs are open, honest and authentic, according to Debbie Weil, a former journalist and Internet marketing consultant who is author of "The Corporate Blogging Book" (Penguin, 2006).

"Packaged, filtered, controlled conversations are out," Weil says. "Open, two-way, less-than-perfect communications with your customers and employees are in." Weil, who blogs about business blogs at www.debbieweil.com, says corporate bloggers need to write with a human voice, post often, be transparent and enter into dialog with readers.

It turns out that there's no one way of doing a corporate blog. A few are written by CEOs like Schwartz or high-ranking executives like vice chairman Bob Lutz of General Motors (Charts), who gets help from other senior execs at GM. Others are so-called group blogs, like Poliblog, a new entry in which Verizon (Charts) executives discuss technology and telecommunications policy.

Still others range far afield: Wells Fargo (Charts) operates a blog called "Guided By History" which is all about natural disasters and addresses such questions as: "How do you prepare for drought?"

Not surprisingly, technology companies led the way into business blogging. IBM (Charts) and Microsoft (Charts) unleashed thousands of bloggers, many who write for small, specialized audiences. But Robert Scoble of Microsoft broke out of the pack to become a minor celebrity, attracting millions of readers and a contract to write a book about blogs called "Naked Conversations" (Wiley, 2006).

The Economist said of Scoble: "He has succeeded where small armies of more conventional public-relations types have been failing abjectly for years; he has made Microsoft, with its history of monopolistic bullying, appear marginally but noticeably less evil to the outside world."

Good business blogs put a face on impersonal institutions. Dell's bloggers wrote extensively about the recall of lithium ion batteries, providing lots of useful information as well as a feel for what it's like to deal with a massive customer service problem. (They explained that phone lines were overloaded but that Dell's battery-recall Web site had handled 15.4 million visitors.)

McDonald's blog, called Open for Discussion, is written by Bob Langert, a vice president for social responsibility, who writes about obesity, negotiating with Greenpeace and why Mickey D's gave out Hummer toys in its Happy Meals. In an unpredictable blog called Nuts About Southwest, a pilot writes about how he spots thunderstorms on radar and a flight attendant describes the joys of staying over in San Diego.

There are, of course, risks to corporate blogging. Dell's site was originally called "one-2-one" until a reader pointed out a porn site of the same name; it's now called "Direct2Dell." Boeing's blog looked like a PR operation when it all but ignored the news that the company's CEO, Harry Stonecipher, was forced to resign after having an affair with a subordinate.

Still, as companies learn how to use blogs well, we can expect to see many more of them. "If you don't talk with your employees and your customers in a way that lets them in on things, you're missing out on the game," says Edelman.

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