A while ago I was interviewed by a TV news syndication service for a story on sites that let you sell your expertise online. The story showed, just briefly, screenshots of a few of the services. For completeness' sake, here's a list of 10 sites (plus a few bonus sites) you can use to monetize your down cycles, e.g., make money in your spare time from the stuff in your head.
1. BitWine has a large list of topics from which you can select your expertise. You set your own hourly rates. Calls from customers looking for your expertise come in to your computer, but people calling you can connect to the network via ordinary telephone. (See story.) See also Wengo, a European take on this concept.
2. Ether has a slick system to connect people to your phone line and charge them for the call. Its big difference is that there's no directory. You put a little "call me" widget on your own site or blog, where, as the company says, "trust already exists." (Review.)
3. Skype Prime is like Ether, but works over the Skype Internet Telephone (VoIP) service. Skype takes a large percentage of revenues for using this service (30 percent, compared to Ether's 15 percent). (Story.)
5. oDesk is a marketplace for programmers. It's a fully developed ecosystem where programmers can take courses, get certified, and sell their programming chops either by the hour or the contract. It's great if you're a programmer looking for customers from other countries. (Stories.)
6. ChaCha is a search engine with a human touch: "Guides" help searchers find online resources. It's not a high-paying gig, but if you have time to kill, you might be able to make a few bucks, and you won't even have to open the shades to do so. (Review.)
7. Brijit is a collection of abstracts of current text and video stories. In other words, the CliffsNotes of modern periodical publishing. Who writes all those abstracts? You do. Get $5 a story and $8 for abstracts of videos.
8. eHow is a collection of user-writer guides. The company splits revenue from ads on the pages with the authors. However, you'll probably get the most real value from eHow by learning from its many guides on running businesses. See also HowToDoThings.
9. Uclue lets users submit questions with prices for the answers. Answer questions, earn money.
10. Amazon's Mechanical Turk splits up tasks that computers can't do well into tiny pieces of work that humans can do well, and rewards them for helping out. It's rote work, like tagging photographs with the names of objects in them, but if you get into a rhythm in pennies will roll right in.
Summary: If you have a day job, don't quit. But if you're looking for a modest supplemental income, you don't have to leave the house to get it.