My odds:
Charlie Crist 5 to 1. Might be to effeminate to appeal to "values voters", but on the plus side, last name easily confused for "Christ" so evangelicals may think Jesus is running with McCain. Also governor of an important swing state.
Tim Pawlenty 6 to 1. Minnesota's not going for McCain, but Pawlenty has the advantage of not being too charismatic on the bottom of the ticket and is on the right side of moron issues like requiring the pledge of allegiance in classrooms. He's like a little too liberal on many issues to do McCain mcuh good with his base, and there are probably better options if he's going to tack towards the center for the General. For instance:
Tom Ridge 4 to 1 Moderate, in line with McCain's politics for the most part. Pro Choice, meshes well with the fear message having been head of Homeland Security. Has good name recognition. On the other hand, somewhat responsible for making airports suck. Might pull some votes in Pennsylvania, but likely not enough to really put it in play.
Mark Sanford 2 to 1 I think this is the guy. Conservative, appeals to racist southerners. In a good position to suppress the black vote in SC, lies constantly. Ideal to turn out the base.
The Field 20 to 1 Everyone else. I think it's pretty unlikely there'll be a surprise here. It's well known that McCain just does whatever his advisers tell him to, and I imagine that'll be to choose Sanford this time. I'm not sure if he gets the son and the junkyard in the bargain or not.