Jill Lawrence writes about White House '08, and comes up with quite a long list of names:

Republicans:
(In order of appearance)

Bill Frist

Rudy Giuliani

John McCain

Arnold Schwarzenegger

George Allen

Mitt Romney

George Pataki

Chuck Hagel

Democrats:

John Edwards

Howard Dean

Hillary Clinton

Mark Warner

Phil Bredesen (TN Gov.)

Mike Easley (NC Gov.)

Rod Blagojevich (IL Gov.)

Janet Napolitano (AZ Gov.)

Bill Richardson

Tom Vilsack

Evan Bayh

Jennifer Granholm (MI Gov.)