PollingReport
http://www.pollingreport.com/2008.htm
The PollingReport complies all the polls from national surveys that are being conducted. It is an independent, non-partisan site.
National Council on Public Polls (NCPP)
The NCPP sets professional standards for public opinion polls. It provides information, including an article for journalists, entitled, "20 Questions A Journalist Should Ask About Poll Results."
Pollster
This site allows you to quickly scan the latest trend poll trend estimates for every state in the Presidential race.
Real Clear Politics
http://www.realclearpolitics.com/polls/
This site is updated daily and provides a number of political polls for national and state levels.
American Research Group, Inc.
http://www.americanresearchgroup.com/
National polling service.
Gallup
http://www.gallup.com/Home.aspx
National polling service.
Rasmussen Reports
http://www.rasmussenreports.com/
National polling service.
Zogby
National polling service.
Pew Research Center for the People & the Press
National polling service that goes beyond just politics. Also see Pew Social Trends and Pew Global Attitudes Project.
Quinnipiac University Polling Institute
http://www.quinnipiac.edu/x271.xml
A polling institute that covers the Northeast region primarily.
Marist Insitute for Public Opinion
http://www.maristpoll.marist.edu/default.htm
A survey center at Marist College