The American electoral system is heavily biased against more than two parties, just as all winner take all, single-member constituencies are. It is not because of money, entrenched corruption or media conspiracy (though the two parties having the lion's share of the money is certainly an effect). It is also not uniquely American, other nations with this way of electing representatives have also not had histories of successful third parties.
There are several alternatives such as a proportional system i.e. Italy, however these are prone to instability and rather bizarre coalitions. Our system was intentionally created for stability, slow change, and compromise--a system that would allay the fears of the states and at the same time prevent, however imperfectly, the states from destroying each other and which except for one horrible moment has worked very well.
Unlike a parliamentary system, in our nation, compromise, change and coalition building, which provide the electorate with their alternatives, is primarily created before the elections. Where ideas, ideals, and agendas change is in the process that occurs before the election. If you want to have more of a voice in this, other than simply choosing between two options, is to get involved in the primary process and earlier, within the party that most closely aligns with your ideals--and remember that they are ideals and you have to compromise to get anything done, not compromise your soul away but fight and work where you can and get the results that are within reach.
Remember also, this same system which makes it frustratingly difficult to get you ideals into policy, also works to frustrate the objectives of the other side. There is no accident that the congress of the past two years has done very little, they have the majority, but they can't get sweeping changes done, that is the way our system is set up--to either force compromise or settle for inactivity. The alternative is sweeping radical changes in our daily lives at each election cycle. I am guessing from the purpose of this site, that many readers would consider the change that would be occurring now as the result of the 06 elections would be catastrophic if the above was not the case. To paraphrase Churchill, 'its the worst system created by man, except for all the others'.
The last successful third party was the republican party and it was successful because the other parties fractured over the most horrible moment in our history(Lincoln ran against three other major opponents, opponents with either only regional support or deeply compromised as in Douglas). Other than that, every instance of a successful third party has only succeeded in causing one party to loose, and that loosing party has always been the one most closely allied in principle to that third party. Further, any lasting impact of that third party bid, at the next election cycle will be absorbed by the larger party. True changes to this state of affairs would require drastic revisions of our constitution.
If dissatisfied with the way things are now, get out and shake hands, smile, put on a suit, convince neighbors, volunteer, give money to candidates, convince others to give money, volunteer to be a delegate at state and national level and a myriad of other opportunities and you will have an effect on the contrasting alternatives offered by the two parties. However, voting for a third party may have the opposite effect to the one you are seeking.