The index seems almost infinite. Emily's List is a group which promotes women's issues and candidates. The Independent Institute reviews a broad range of issues, and also provides many unique links. National Center for Policy Analysis provides a number of links to political issues from a conservative perspective. It's like an index, but a good one.
Propaganda is a very interesting site.
Not only does the site define the various types of propaganda, but includes links to actual examples from Speaker Gingrich to the Democratic National Committee. Public Agenda examines issues and provides briefing reports. Rock the Vote is for the MTV generation.