Remagi and I have had several "discussions" about third party candidates. I tend to agree with Humwuwu.
I would really, really like to agree with Rathic. I believe it was Thomas Jefferson that warned us against going with a two-party system. However, that doesn't change the fact that we will never adopt another system (not in my lifetime anyway).
Say I am voting for Ralph Nader (I actually briefly visited his website). I didn't have time to read in depth on where he stands on the issues, but it doesn't seem too extreme. I don't agree with his stance on nuclear power though. Or I guess it's nukular power if your name is Sarah Palin.

So just pretend that I love Nader, and I totally want him to be president, so as a fiscally conservative, socially liberal voter, I cast my vote for him.
The only problem is, he isn't going to win. I know he's a great guy and all, and has a vision for the US, but he's going to lose, period. He doesn't have the finances to compete with the 2 major party candidates, he isn't invited to the debates, it sucks but that's the way it is.
So because I live in the real world, and I realize this for truth, I can either vote for McCain or I can vote for Obama. Essentially, a vote for Nader is a vote for McCain. It happened in 2000 when Gore lost to Bush. Just think of how different the world might be if Nader had not have run in 2000. He received 2.74% of the popular vote, and I think we all remember how that election ended. If you ask a Democrat, they will tell you it was Nader's fault, If you ask a Green Party member, they will tell you that it wasn't. So who knows? It also works the other way around. When I was a 19 year old Private in AIT in 1992, Bill Clinton was elected president. Now, wikipedia says, "Exit polls also showed that Ross Perot drew 38% of his vote from Bush, and 38% of his vote from Clinton, while the rest of his voters would have stayed home in his absence on the ballot," but I have heard people argue that it is Perot's fault that H.W. lost the election. Is it true? I guess we'll never know.
Anyway, enough rambling. I think Rathic and Humwuwu both have valid points. On one hand, it's a matter of principle to cast a protest vote for a third party. On the other hand, it's a vote for the Conservative when you vote for the third party liberal.
Didn't you ever see that Southpark? It's almost always between a Giant Douche and a Turd Sandwich.