Well, looks like we're almost exclusively discussing only Democratic and Republican hopefuls.
According to
wiki, the following people already entered the race under third parties/indy banner:
Green Party:Stewart Alexander (2008 Socialist Party VP nominee)
Kent Mesplay (Green National Committee member)
Libertarian Party:R. Lee Wrights (Libertarian National Committee Member)
Prohibition Party:Jack Fellure (already a nominee, a perennial candidate from WV, who has been running in every GOP primaries since 1988)
Independents:Naked Cowboy from New York
Joe Schriner (perennial candidate from OH)