I really like oHours.org.
Nate started the company a while back, and seems to be focused on his other startup, but to me this is the basis for a truly great experts network that gives them the ability to monetize their excess capacity. In other words, to be able to charge people for consulting hours.
I asked him a year ago when he was going to add in payments, and he pretty much said he wasn't thinking about it. The payment system still isn't in there, but it's easy enough to send someone to paypal (ugh) or some other interface to charge for the hours.
But there should be an option to require prepayment, so your time as an expert isn't wasted.
Today I have slots available starting at 1:30. They are pretty brief at 20 minutes, and free. You'll notice the little oHours widget at the top of the right column, just about another cool widget for searching the site and its comments. I'll blog about Gawk.it after I play with it a bit more.