Here's a full list:
candidates for 2008
Democrats
Evan Bayh, U.S. Senator and former Governor, Indiana
Rod Blagojevich, Governor of Illinois
Bill Bradley, former U.S. Senator, New Jersey; 2000 nomination candidate
Hillary Clinton, U.S. Senator, New York
Wesley Clark, retired Army General and former NATO Supreme Allied Commander; 2004 nomination candidate
Jon Corzine, U.S. Senator, New Jersey
Richard M. Daley, Mayor of Chicago
Tom Daschle, U.S. Senate Minority Leader, South Dakota
Howard Dean, former Governor of Vermont; 2004 nomination candidate Presidential nomination
Richard Durbin, U.S. Senator, Illinois
John Edwards, 2004 Vice Presidential nominee, and outgoing U.S. Senator for North Carolina (as of 2004); 2004 nomination candidate
Russ Feingold, U.S. Senator, Wisconsin
Harold Ford, Jr., U.S. Congressman, Tennessee
Al Gore, former Vice President and 2000 Presidential nominee
Joe Kernan, Governor of Indiana
John Kerry, 2004 Presidential nominee and U.S. Senator, Massachussets
Mary Landrieu, U.S. Senator, Louisiana
Blanche Lincoln, U.S. Senator, Arkansas
Janet Napolitano, Governor of Arizona
Gavin Newsom, Mayor of San Francisco
Barack Obama, Democratic candidate for U.S. Senate in Illinois (as of 2004)
Nancy Pelosi, U.S. House of Representatives Minority Leader, California
Ed Rendell, Governor of Pennsylvania
Bill Richardson, Governor of New Mexico
Charles Schumer, U.S. Senator, New York
Eliot Spitzer, Attorney-General of New York
Mark Warner, Governor of Virginia
Republicans
George Allen, Jr., U.S. Senator, Virginia
John Ashcroft, U.S. Attorney General and former U.S. Senator, Missouri
Haley Barbour, Governor of Mississippi
George W. Bush, current President, will be eligible if he loses in 2004 (see also Possible Constitutional amendments below)
Jeb Bush, Governor of Florida
Elizabeth Dole, U.S. Senator, North Carolina; 2000 nomination candidate
Bob Ehrlich, Governor of Maryland
Bill Frist, U.S. Senate Majority Leader, Tennessee
Rudy Giuliani, former Mayor of New York City
Lindsey Graham, U.S. Senator, South Carolina
Chuck Hagel, U.S. Senator, Nebraska
Kay Bailey Hutchison, U.S. Senator, Texas
John McCain, U.S. Senator, Arizona; 2000 nomination candidate
Bill Owens, Governor of Colorado
Rick Perry, Governor of Texas
George Pataki, Governor of New York
Colin Powell, U.S. Secretary of State
Dan Quayle, former Vice President; 2000 nomination candidate
Condoleezza Rice, National Security Advisor
Tom Ridge, U.S. Secretary of Homeland Security and former Governor of Pennsylvania
Mitt Romney, Governor of Massachusetts
Gordon Smith, U.S. Senator, Oregon
Tommy Thompson, U.S. Secretary of Health and Human Services and former Governor of Wisconsin
Greens
Peter Camejo, businessman
David Cobb, lawyer and activist, and nominee for President in 2004
Matt Gonzalez, president of the San Francisco Board of Supervisors
Winona LaDuke, activist and nominee for Vice President in 1996 and 2000
Pat LaMarche, activist and nominee for Vice President in 2004
Libertarians
Michael Badnarik, 2004 Presidential nominee
Michael Cloud, speechwriter
Michael Colley, retired Navy Admiral
James Gray, judge
Carla Howell, management consultant
Gary Nolan, radio host
Ron Paul, Republican Congressman, Texas
Aaron Russo, entertainment businessman
L. Neil Smith, science fiction writer
Ed Thompson, former Mayor of Tomah, Wisconsin
Other parties and independents
Pat Buchanan, America First, commentator and former Presidential candidate
Roy Moore, Constitution Party, former Chief Justice of the Supreme Court of Alabama
Ralph Nader, activist and former Presidential candidate
Jesse Ventura, former Governor of Minnesota and Professional Wrestler It is also conceivable that a candidate for a major party nomination that did not win that nomination might seek the presidency as an other party or independent candidate; contemporary examples include John Anderson, Republican nomination candidate and Independent general election candidate in the 1980 election, and Buchanan, Republican nomination candidate and then Reform Party general election candidate in the 2000 election.
Possible Constitutional amendments
There has been some discussion recently of amending the Constitution to remove the absolute requirement that only natural-born citizens may become President. If such a change occurred in time for the 2008 elections, possible candidates who are naturalized citizens would include Democratic Michigan Governor Jennifer Granholm, born in Canada, Democratic former Secretary of State Madeline Albright, born in Czechoslovakia, and Republican California Governor Arnold Schwarzenegger, born in Austria.
The possible repeal or amendment of Amendment XXII has also come under some discussion. One proposal would allow a President to serve only two consecutive terms but seek to return after a respite. The only living former president who has served two terms is Bill Clinton. If George W. Bush won re-election in 2004, and sentiments across party lines looked favorably on a contest between him and Bill Clinton in 2008, a change to this term limit might find support.