Republican
Tim Pawlenty from Minnesota - Gives McCain a running mate from the northern states. Plus, he is younger than McCain and he's a Governor, not a Senator. If I was betting money I would put money on Pawlenty.
Mitt Romney - He's doing a good job of campaigning for McCain. The folks here in Colorado votoed for Romney in the caucus in droves. I couldn't believe it.
John Thune - Another Senator, but from South Dakota. The only state less likely to produce a VP is Wyoming. Wait, isn't Cheney from there?

Joe Lieberman - He and McCain get along well personally. But Lieberman is way too liberal (not Obama Liberal though) and conservatives would revolt.
Carly Fiorina - Fired CEO of Hewlett Packard. I don't see this helping much unless McCain is looking to gender balance the ticket. Given that HP went downhill while Fiorina was a high paid CEO I think this is too much of a negative.
Democrats - I don't know this group too well but here goes...
Hillary Clinton - Put Hillary and Barack together and it is uncertain if primary voters that supported either of them can hold their nose for the other. Still, this is a ticket that would be hard to beat if they can stand each other.
Joe Biden - Joe has a problem with saying the wrong thing at the wrong time. Plus, he's another Senator.
Al Gore - He may be green, but he has experience.
Tim Kaine - Early supporter of Obama but it leaves too much of the West uncovered.
Jim Webb - A new Senator from Virginia he has military experience. But, once again, being from the East, he leaves the West uncovered.
John Edwards - The pretty boy of former Presidential contenders didn't have enough oomph in the primary and he is unlikely to help beyond his populist draw.