From the outside it sounds like there are only two political parties in the USA, but you tend to hear of many others in other countries. Are there others that just don't get votes?
However, the First-Past-The-Post electoral system (AND in addition, electoral college as noted in Affable Geek's excellent answer here) in US makes any party outside Republicans/Democrats to be largely not viable as far as practical electability.
This isn't always the case and in some exceptional circumstances a 3rd party emerges, frequently as a result of splintering of one of the two major parties.
Since the collapse of the Whig Party in the 1850's, every US president has been either a Democrat or a Republican.
There have, however, been times in the past when third parties have done better:
- The Populists got 8.5% of the popular vote in the 1892 presidential election, and went on to win some governorships and Congressional seats.
- The Progressives came in second in the 1912 presidential election as Theodore Roosevelt (formerly a Republican) made a run for a third term. They did succeed in electing a California governor and some other state and local offices.
- The Dixiecrats won a few Southern states in 1948.
- The American Independent Party, another segregationist party, won the Deep South in 1968.
- Ross Perot, founder of the Reform Party, won 18.9% of the popular vote in 1992.