Q&A site for programming questions - StackOverflow has really taken off from the beginning of 2009. It's now Alex 1,570, which is extremely high for such a niche site (of course, the intended audience is all using the Alexa toolbar!).

They have an extensive user karma/reputation system to try to control spam.

The mechanical Turk-based Q&A service for Kindle users has been discontinued. Amazon is now promoting Askville.com.

Amazon's "answers" site. Is this the same one that Kindle owners got access to when the 1.0 Kindle was released?

Quoted: Do you need help or need to ask a question? Can't find it on Search? Askville is a community where people love helping others by answering questions.

Earn $5 by answering my Mahalo Answers question.

I haven't used Yahoo Answers yet - but it seems like it's doing very well (and one of the few places Google is being handed their hat even when they had a very early entrant to the market).

Now Yahoo is adding a social networking component ("in beta") to help you stay connected with experts you count on.

Quoted: Yahoo Answers is a bright spot for the company. Archrival Google had a competing service called Google Answers that never took off and was put out to pasture last year. In late December of last year, market researcher Hitwise Pty reported that, among Q&A-type search engines, Yahoo Answers held a 96 percent of share of visits in the U.S. Answerbag ranked second with 2.04 percent and Microsoft's Windows Live Q&A came in third with 1.02 percent.

Guided search site.

Quoted: Click above for A Community for ChaCha Guides on ChaCha