Ross Perot's third-party efforts in 1992 and 1996 were the last independent or third-party presidential campaigns in American politics which met with even spoiler-level success. Before that, it was Theodore Roosevelt at the beginning of the twentieth century. So we're not due for another such impact for about sixty or seventy years or so, barring major electoral realignments or new movements which seem still beyond the horizon.
Nevertheless, there is hardly ever a lack of those willing to try. In 2008, no independent or third-party effort challenged the primacy of the two parties, and there are no major stars on the horizon for 2012 so far.
Goode / Clymer
The Constitution Party is aiming for ballot access in all 50 states for the 2012 presidential election. Their national convention was held in Nashville, Tennessee, and on April 21, 2012, nominated Fmr. Rep. Virgil Goode of Virginia as their candidate for president, and Party Chairman Jim Clymer of Pennsylvania as their vice presidential candidate.
Johnson / Gray
The Libertarian Party confirmed ballot access in 29 states for the 2012 presidential election. Their national convention was held in Las Vegas, Nevada, and on May 5, 2012, nominated former 2012 Republican presidential candidate Fmr. Gov. Gary Johnson of New Mexico as their candidate for president, and anti-drug law activist and former Orange County, California, Superior Court Judge Jim Gray as their vice presidential candidate.
Stein / Honkala
The Green Party confirmed ballot access in 21 states for the 2012 presidential election. Their national convention was held in Baltimore July 12-15. They nominated internist Dr. Jill Stein, M.D. (MA), for president and anti-poverty activist Cheri Honkala (PA) for vice president.