Following the tradition that Marvel has made so popular, Warner Bros has included a very special post credits scene at the end of Green Lantern. Check out what Sinestro himself, Mark Strong, had to say about it.
“That was shot speculatively, I think. There was an idea that there would be an introduction of the idea that the yellow ring corrupts him. Perhaps it was felt in the body of the film, it wasn’t the right time to introduce it. So actually the thing to do is introduce everybody to the mythology and the story, and then give them that little taste. Certainly the fanboys who know about it will be excited, and anybody else, at least they have an opportunity of understanding where it could go from here.”