http://gp.org/ - Green Party of the United States
http://www.airamericaradio.com/ - Air America online Radio
http://mediamatters.org/ - Researches misinformation in the U.S. media.
http://www.stateaction.org/ - Center for Policy Alternatives: Progressive/Liberal resource for sample legislation and issues research
There are many other excellent liberal and progressive sites, but rather than listing all of them here, the following two sites have lengthy lists of links
http://liberalforum.org/ - Discussion forum for liberal issues
http://www.liberaloasis.com/ - News and views from a liberal perspective