The Libertarian party favors a smaller government and free markets with minimal regulations. Although sometimes regarded as conservative, on many social issues the party is closer to liberals, espousing strong civil liberties, drug liberalization, and LGBT rights. On foreign policy the party is for less intervention and favors diplomacy rather than military might to settle disputes with other countries.
The U.S. Green Party has nominated Jill Stein as their candidate after a convention in Baltimore on July 12th this year. The Green Party stresses environmentalism. However the party also promotes participatory democracy, social justice, respect for diversity, peace and non-violence. Their ten key values can are outlined here.
The Socialist Party USA is a third alternative party. This is a democratic socialist party opposed to communism and bureaucratic types of top-down socialism. Stewart Alexander became the nominee for the party back in October of 2011 at a convention in Los Angeles. The party advocates bringing big business under public ownership and democratic worker management. The website of the party can be found here.