Two summers ago, the extent of Arctic sea ice reached a historic low. What's it going to do this year?
Chuck at Lounge of the Lab Lemming has opened a betting pool. No money involved; winner gets to make him blog on a topic of the winner's choice. (This is a good prize; Chuck is funny.) Bets are in the form of extent and uncertainty, and Chuck's got a useful graph that illustrates how he plans to judge the winner. (It also includes all the sea-ice minima since 2002 in a handy graphical form, which is useful for those of us who prefer pictures to words and numbers.)
He's also interested in seeing how the guesses are distributed given different backgrounds (climatologist, non-climatologist, random friend). So go pick a number, any number, and see how you do.