At intrade.com, people bet real money on a variety of non-sporting events. Most of the events are politics, particularly who will win elections.
As it stands, here are the percentages given to the main GOP candidates currently:
Mitt Romney: 73%
Rick Santorum: 13.2%
Newt Gingrich: 3.7%
Ron Paul: 2.8%
Sum it up and the prediction markets indicate about a 92.7% chance that the nominee is one of the four candidates currently actively campaigning. Keep in mind, these percentages include the possibility of one of these candidates winning the nomination through a brokered convention. Bettors are giving at least a 7% chance that not only will there be a brokered convention, but the ultimate nominee will be someone who is not even actively campaigning.
Of that, most of the bets are on Jeb Bush being the nominee, though I personally can’t imagine why the Republicans would nominate another Bush. Perhaps the only thing Obama wants more than a Santorum nomination is to be able to actually run against a Bush.