I do not claim great knowledge about third parties, but here in New York City there is the Working Families Party.
>From their website I got the following:
Unlike most other third parties, we think that constantly "spoiling" elections -- i.e. voting for third party candidates with no chance of winning -- is not a smart strategy. Too often, it pulls votes from a somewhat desirable candidate, causing the least desirable candidate to win. Using fusion, we support good candidates who can win, and use the number of votes we get for them on our line to exercise real power over the decisions they make once in office. And within party primaries (usually Democratic), we use our unbeatable grassroots organizing skills to make sure the better candidates win.
I know this would be anathema to those who regard the Democratic Party as the devil's spawn, but I believe it is a smart way to leverage power. I think it both builds a party and influences the Democrats.
Brian Dauth Queer Buddhist Resister