First was the big green button. More than 10 years ago charities and charitable sponsors started using it and soon it was everywhere. The text read: “Click here and we will donate 1p for each click”. Nowadays, a Tweet does just as well. It’s simple, instant and rewarding both for the service/product provider and the audience.
Here is an example from Twittercounter
And here are some other examples of how Pay with a Tweet is being used: http://www.paywithatweet.com/cases.html