As much as I disagree with them, I think the Tea Party at least originated as a third party, olddude. You can certainly argue that they are really an offshoot of the Republicans, but there were quite a few primaries where the Republican Party's chosen candidate was eliminated by a Tea Party candidate. As a result of their activity, the Republican Party has again been forced to run to the right of its traditional base.
The Tea Party does display a couple of features of a true third party. It is primarily a grassroots effort, rather than a party designed around an individual running for the Presidency, like Nader and Perot's "third parties". It doesn't have a dominating figurehead, although Palin has been adopted as a favorite, but is based more on a philosophy of government.