Most viable third parties?

[AltWorlder]

I know that at present, one of the existing U.S. third parties unseating the big 2 is very unlikely.  Regardless, which of the 3rds are the most viable and likely to exist for the foreseeable future as the top ones?

I think it's the big 3- Libertarians, Greens, and Constitution.  I guess it makes sense because the Greens are a bit far left (and outright Socialist parties are immediately stigmatized in the U.S.), the Constitution Party is rather far right (albeit paleocon and theocon), and the Libertarians occupy one point of the political compass that isn't explicitly represented by the Dems or the GOP.  I think that there may be a Christian Democrat party in the future to occupy the opposite compass point, assuming the Republicans don't outright splinter a CDP out, but I'm guessing the Libs, Greens, and Cons will be here to stay.  The Reform Party basically broke up, and I'm not sure who's out there of note as far as third parties go.

[AltWorlder]

I think the two you named certainly get props for staying power, but I was thinking more about national parties for this thread.

[AltWorlder]

Slight alternate history question: What could have become the fourth (and fifth) major third parties in the U.S.?  Could the Reform Party have survived instead of being broken up?  I can't think of any other ones, but that page lists basically all of the big ones in the U.S.  I was going to suggest the Socialists, but they were pretty much tarred with the Cold War.