Further information: Political parties in the United States
Each of five parties shares a degree of national attention by attaining the mathematical possibility of its nominee becoming President of the United States -- i.e., having ballot status for its presidential candidate in states whose collective total is at least half of the Electoral-College votes -- in the U.S. presidential election, 2008.
Democratic Party (1828 modern, 1792 historic)
Republican Party (1854)
Libertarian Party (1971)
Constitution Party (1992)
Green Party (1996)
 Other parties that have nominated candidates in recent elections
These parties have offered candidates in recent elections, but did not in 2004, and they do not have ballot status in enough states in 2008 to win the presidency. Some do not have presidential candidates, but for other offices only.
This is an incomplete list, which may never be able to satisfy certain standards for completeness. Sourced additions are welcome and you can help by expanding it.