Please note that in current American parlance, "Progressive" does not necessarily equate to "Democrat". Progressives are at a point in the political map to the left of Liberals, whom Progressives consider as lacking the will for essential changes in society. However, while according to the old dichotomy Progressives would be on the "Left", they often would not align themselves with the rigid ideological frameworks of Socialism or Communism.
To take up the British method of defining political positions with periodicals, (e.g. New Statesman for Labour, The Spectator for Conservatives) here's a sampling of the American alternative press
Liberal: http://www.thenation.com/
Progressive: http://www.progressive.org/,
Radical: Z -Magazine http://www.zmag.org/
Socialist: 'Democratic Left' - http://www.dsausa.org/dl/index.html
Communist: http://www.peoplesworld.org/
