You are a world-class expert. Yes, you are, just like Stephen Hawking is a world-class expert on the Big Bang and Martha Stewart is a world-class expert on making festive party favors out of tissue paper.

Your personal interests, life and work experiences and education add up to real expertise in a number of subjects. Do you tinker with cars? Read Shakespeare for fun? Beat your buddies at chess? Lecture your friends about proper diet, or Web design, or Star Wars collectibles?

Then you’re an expert, and there’s a place online that can help you turn that avocation into hard cash.

There are at least three types of Web-based businesses that can help you earn cash in your spare time -- and a fourth type you shouldn’t go near.

All three of the legit types supply the online base, the software and the marketing muscle you need to get started. You supply the expert “content,” to use the hideous neo-media word for what we call “facts” in plain English.

This is not the get-rich-quick scheme you may have been looking for. The people who make real money through any of these schemes are working hard for it, really know their subjects and are good at communicating.

The money they earn is based on hard numbers. Page views and user ratings are modern equivalents to piece-work counts in a factory.

But if you can’t find a full-time job, can’t work regular hours or outside your home, or want to moonlight to supplement the 9 to 5 grind, one of these might be for you. For more strategies on how to moonlight, see Ten Ways to Moonlight.

A look at some of these online businesses suggests they are being used by young and out-of-work professionals to build up their resumes and hone their skills until the economy recovers. And, of course, many managers are using the services to get the job done until they can afford to add headcount.

All the sites claim to conduct a vetting process for contributors, and quality control over contributions, but their criteria are positively loosey-goosey compared to the personnel departments of a major corporation.

After all, their system of paying based on actual usage and client ratings ensures that only really good contributors get paid, and bad ones sink into the unplumbed depths of the database.