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.)