Here's my tentative list of candidates for each election. For 1789-1800, which was before electors had to differentiate between votes for president and VP (and popular vote tallies seem to be rather lacking), I included everyone who received at least 5% of the electoral vote. From 1804-2008, I included everyone who received at least 0.05% of the popular vote. I made this cutoff low so as to spice up the elections with some wacky third-party candidates. What do you think?
1789 (6): George Washington, John Adams, John Jay, Robert Harrison, John Rutledge, John Hancock
1792 (3): George Washington, John Adams, George Clinton
1796 (7): John Adams, Thomas Jefferson, Thomas Pinckney, Aaron Burr, Samuel Adams, Oliver Ellsworth, George Clinton
1800 (4): Thomas Jefferson, Aaron Burr, John Adams, Charles Pinckney
1804 (2): Thomas Jefferson, Charles Pinckney
1808 (3): James Madison, Charles Pinckney, James Monroe
1812 (3): James Madison, DeWitt Clinton, Rufus King
1816 (2): James Monroe, Rufus King
1820 (2): James Monroe, DeWitt Clinton
1824 (4): John Quincy Adams, Andrew Jackson, William Crawford, Henry Clay
1828 (2): Andrew Jackson, John Quincy Adams
1832 (3): Andrew Jackson, Henry Clay, William Wirt
1836 (4): Martin Van Buren, William H. Harrison, Hugh White, Daniel Webster
1840 (3): William H. Harrison, Martin Van Buren, James G. Birney
1844 (3): James K. Polk, Henry Clay, James G. Birney
1848 (4): Zachary Taylor, Lewis Cass, Martin Van Buren, Gerrit Smith
1852 (5): Franklin Pierce, Winfield Scott, John Hale, George M. Troup, Jacob Broom
1856 (3): James Buchanan, John Fremont, Millard Fillmore
1860 (4): Abraham Lincoln, John Breckenridge, John Bell, Stephen Douglas
1864 (2): Abraham Lincoln, George McClellan
1868 (2): Ulysses Grant, Horatio Seymour
1872 (4): Ulysses Grant, Horace Greeley, Charles O'Conor, James Black
1876 (4): Rutherford Hayes, Samuel Tilden, Peter Cooper, Green Smith
1880 (4): James Garfield, Winfield Hancock, James Weaver, Neal Dow
1884 (4): Grover Cleveland, James Blaine, John St. John, Benjamin Butler
1888 (4): Benjamin Harrison, Grover Cleveland, Clinton Fisk, Alson Streeter
1892 (5): Grover Cleveland, Benjamin Harrison, James Weaver, John Bidwell, Simon Wing
1896 (6): William McKinley, William Jennings Bryan, John Palmer, Joshua Levering, Charles Matchett, Charles Bentley
1900 (6): William McKinley, William Jennings Bryan, John Woolley, Eugene Debs, Wharton Barker, Joseph Maloney
1904 (6): Theodore Roosevelt, Alton Parker, Eugene Debs, Silas Swallow, Thomas Watson, Charles Corregan
1908 (7): William Taft, William Jennings Bryan, Eugene Debs, Eugene Chafin, Thomas Hisgen, Thomas Watson, August Gilhaus
1912 (6): Woodrow Wilson, Theodore Roosevelt, William Taft, Eugene Debs, Eugene Chafin, Arthur Reimer
1916 (5): Woodrow Wilson, Charles Hughes, Allan Benson, James Hanly, Arthur Reimer
1920 (7): Warren Harding, James Cox, Eugene Debs, Parley Christiansen, Aaron Watkins, James Ferguson, William Cox
1924 (7): Calvin Coolidge, John Davis, Robert La Follette, Herman Faris, William Foster, Frank Johns, Gilbert Nations
1928 (6): Herbert Hoover, Alfred Smith, Norman Thomas, William Foster, Verne Reynolds, William Varney
1932 (7): Franklin Roosevelt, Herbert Hoover, Norman Thomas, William Foster, William Upshaw, William Harvey, Verne Reynolds
1936 (6): Franklin Roosevelt, Alfred Landon, William Lemke, Norman Thomas, Earl Browder, D. Leigh Colvin
1940 (5): Franklin Roosevelt, Wendell Willkie, Norman Thomas, Roger Babson, Earl Browder
1944 (5): Franklin Roosevelt, Thomas Dewey, Norman Thomas, Claude Watson, Edward Teichert
1948 (7): Harry Truman, Thomas Dewey, Strom Thurmond, Henry Wallace, Norman Thomas, Claude Watson, Edward Teichert
1952 (4): Dwight Eisenhower, Adlai Stevenson, Vincent Hallinan, Stuart Hamblen
1956 (5): Dwight Eisenhower, Adlai Stevenson, T. Coleman Andrews, Eric Hass, Enoch Holtwick
1960 (6): John Kennedy, Richard Nixon, Eric Hass, Rutherford Decker, Orval Faubus, Farrell Dobbs
1964 (3): Lyndon Johnson, Barry Goldwater, Eric Hass
1968 (6): Richard Nixon, Hubert Humphrey, George Wallace, Henning Blomen, Dick Gregory, Fred Halstead
1972 (6): Richard Nixon, George McGovern, John Schmitz, Linda Jenness, Benjamin Spock, Louis Fisher
1976 (9): James Carter, Gerald Ford, Eugene McCarthy, Roger MacBride, Lester Maddox, Thomas Anderson, Peter Camejo, Gus Hall, Margaret Wright
1980 (6): Ronald Reagan, James Carter, John Anderson, Edward Clark, Barry Commoner, Gus Hall
1984 (7): Ronald Reagan, Walter Mondale, David Bergland, Lyndon LaRouche, Sonia Johnson, Bob Richards, Dennis Serrette
1988 (5): George Bush, Michael Dukakis, Ron Paul, Lenora Fulani, David Duke
1992 (6): William Clinton, George Bush, Ross Perot, Andre Marrou, James Bo Gritz, Lenore Fulani
1996 (7): William Clinton, Robert Dole, Ross Perot, Ralph Nader, Harry Browne, Howard Phillips, John Hagelin
2000 (7): George W. Bush, Al Gore, Ralph Nader, Pat Buchanan, Harry Browne, Howard Phillips, John Hagelin
2004 (6): George W. Bush, John Kerry, Ralph Nader, Michael Badnarik, Michael Peroutka, David Cobb
2008 (6): Barack Obama, John McCain, Ralph Nader, Bob Barr, Charles Baldwin, Cynthia McKinney