During the Credits
There is an extra scene about two minutes into the credits, check out more details and spoilers below.
The extra scene flashes back to Oa and shows the yellow ring encased inside a green protective bubble. The bubble is then broken and Sinestro is shown putting the ring on and his uniform turning yellow.
After the Credits
There is an ad to buy Green Lantern comic books, but there are no real extras after the credits of Green Lantern.
Special thanks to Yakulto, koolaidwino, and AARON WILLS for the stinger submissions!