100 Milestone Documents: Digital images of 100 important documents in US history, ranging from the Declaration of Independence to the Voting Rights Act of 1965.
Factcheck.org: from the Annenberg Public Policy Center
Politifact: from the Tampa Bay Times; rates statements from "true" to "false" (and "pants on fire")
Politico: an online journal about politics and government
Policy Archive: an "open digital archive of foundation-funded and other public policy research"
Pew Research Center: the think tank also conducts opinion polls
Gallup: the consulting and data analytics company is known to most Americans for its opinion polls
FiveThirtyEight: the famous data analysis and statistical site is now owned by ABCNews
FollowtheMoney.org: a good source on campaign contributions (for 2010 and later)
Amnesty International: the famous human-rights organization has a database of countries and regions
Freedom House: an American organization that promotes democracy and human rights; its reports on the state of democracy are frequently cited in the news
Human Rights Watch: publishes articles, reports, and videos on human rights abuses around the world
Witness for Peace/Solidarity Collective: publishes reports, videos, and podcasts on human rights in the Americas
NUKEMAP: see how any location would be affected by a specific nuclear warhead
Living Room Candidate (Museum of the Moving Image): "more than 300 commercials, from every presidential election since 1952"
Constitute: most of the world's national constitutions, translated into English