Many people wants to have a question answered and are willing to pay for it.
Some of these people are students all over the world, who is having a hard time with homework they can't solve and need help!
You'll need to answer 50 questions before you can get to earning. and then they will have you join google adsense.
Lets you earn points which you can redeem for prizes.
You'll need to have 50.000 points to get a 10$ giftcertificate to eg. amazon or walmart. You get 100 points for answering a question and 1000 points for being chosen as an best answer.
There are more points to get depending on what you contribute with.
Is only if you really ARE an expert - why? well, because it's live..
Is again a site for the professionals and only if your question is chosen - They pay you.
Quite new, but looks promising.