So, on Wikipedia there is a great big list of people who had been speculated as Obama's running mate:
http://en.wikipedia.org/wiki/Democr...s,_2008#Fellow_contenders_for_2008_nomination
Now, obviously many of these people are more likely than others, but surprise picks have been made before, so I suppose anyone is possible.
Here are various ways I think the election could have been affected, assuming that McCain keeps Palin as VP:
Chris Dodd and Jack Reed I can't see affecting the election much. Two old northeastern Senators, they seem like they would have the same appeal as Biden.
Hillary Clinton could rally her supporters, but she could also overshadow Obama somewhat, and she has plenty of detractors. In the end, I don't see much of an electoral difference just from her presence.
Ed Rendell would probably just lose Obama votes. I can't see him gaining anything that Biden wouldn't have, and his connections with Norman Hsu will hurt him. Finally, his claims that there was some media conspiracy in support of Obama and his praise of Fox News would turn off many liberals.
Kathleen Sebelius and Brian Schweitzer might be able to gain Obama votes out west, but not a whole lot I fear.
Sam Nunn, Tim Kaine, Mark Warner, Chet Edwards, and Jim Webb could help Obama in the South. Saxby Chambliss and Mitch McConnell could be in danger of being defeated for reelection.
John Edwards could help Obama in the South if he remains scandal-free. If he has a scandal like OTL though, he'd damage the ticket and probably have to be replaced.
Bill Richardson had those pay to play allegations which doomed his OTL Secretary of Commerce nomination, they could hurt him here as well. If he doesn't get brought down by scandal, Richardson could help Obama among Hispanics and the Southwest in general.
Ted Stickland and Evan Bayh could both help Obama in the midwest, but considering he swept that region OTL, wouldn't accomplish much.
Al Gore, Wesley Clark, and James Jones, I don't know.