Actually any of these candidates are still eligible and can be elected should they get the necessary votes.
2012 Presidential Candidates (P2012)
You are not limited to the choice of Democrat or Republican however unless enough people get behind one of the other party's candidate they will just end up siphoning votes off and affecting the outcome by reducing the potential for one of the main candidates to win.
I might actually vote for Gary Johnson if he had a chance of actually winning, while not the perfect candidate either I could live with him much better than the other two choices we face!