Third party politics has long altered the American political landscape. Some of those included, along with periods of most activity:
Anti-Masonic Party (1820s and 1830s)
Liberty Party (1840s)
Free-Soil Party (1840s and 1850s)
Know-Nothing/Nativist Party (1840s-1860)
Greenback Party (1870s)
Prohibition Party (1880s-1890s)
People’s Party (1890s-1900s)
Socialist Party (1900s-19-teens)
Progressive Party (three separate movements: 19-teens, 1920s, 1940s)
For details on each, see this handy Encyclopedia Britannica site. Will the Tea Party change party politics in America? Will it be a factor in the coming years?
Related links: "Third parties leave a mark: A timeline of third party showings," Christian Science Monitor, 28 October 2010.