The U.S. voting system is screwed up. How can Americans have freedom of choice when their choices are limited to two candidates who don't represent them? The Republican and Democratic parties have become so wealthy and powerful that they hedge out any and all competition with little effort. It's not quite a political monopoly, since there are two competing parties. It's an oligopoly.
There are numerous third party groups: The Green Party, The Libertarian Party, The Reform Party, and more. But their candidates are not allowed into the presidential debates, and not included in "equal time" laws.
We're so locked into a two-party system, and compelled to vote for the lesser of two evils that most of us never even consider third party options. CNN has an excellent synopsis of third parties, and a good summary of Ralph Nader.