The American Presidency Project: Archive of data about a wide range of presidential activities such as first year legislation, requests of Congress, major speeches, and approval ratings.
American National Election Studies: Archive of data from national studies on the US electorate.
Gerrymandering Data: Statistical analysis of congressional districts and the level of gerrymandering in each one. Generated from a Princeton study.
Harvard Election Data Archive: Contains data on election results, voting behavior, and electoral politics within the United States.
OpenData Initiative: Archive of data about money in politics and campaign finance.
Voteview: Aggregate of collected data from Professor Keith Poole (UGA) about topics such as political party polarization and congressional voting patterns.