Issues

Compared to the two-headed monster, the "third" parties are so much alike:

Issue Democratic Party Republican Party Constitution Party Libertarian Party Ron Paul Republicans
Follow the Constitution. No No Yes Mostly Yes
Stop trying to be the world's policeman and bring the troops home. No No Yes Yes Yes
Eliminate the income tax and replace it with nothing.  Eliminate the I.R.S. No No Yes Yes Yes
Roll back the welfare state. No No Yes Yes Yes
Phase out Social Security. No No Yes Yes Yes
End the federal reserve and return to sound money. No No Yes Yes Yes
No deficit spending. Pay off the national debt. No Mixed Yes Yes Yes
Vigorously protect civil liberties. Repeal the “Patriot” Act. Mixed No Yes Yes Yes
Protect national sovereignty. No New World Order. No NAU. No No Yes Yes Yes
Vigorously protect property rights. No No Yes Yes Yes
Vigorously protect the right to privacy. Some No Yes Yes Yes
Vigorously protect religious freedom. Some No Partly Yes Yes
Decentralization of power. No No Yes Yes Yes
Vigorously protect gun rights. No Yes Yes Yes Yes
No draft. No No Yes Yes Yes
End government involvement in health care. No No Yes Yes Yes
No wage and price controls. No No Yes Yes Yes
End the War on Drugs. No No No Yes Yes
Proper size of government Bigger A bit smaller Much smaller Much smaller Much smaller
Allow gay marriage Mostly yes No No Yes Yes
Eliminate managed trade. No NAFTA. No GATT. No WTO. No No Yes Yes No
Free trade. No tarrifs. No No No Yes No
Allow open immigration. Mostly No No Yes No
Abortion. Mostly pro-choice Mixed Pro-life Pro-choice Mixed

Key:
Red represents anti-liberty stances.
Green represents pro-liberty stances.
Black represents areas of dispute.

I threw together this chart rather quickly.  I encourage you to let me know where it is inaccurate or incomplete.