Congress
Adam Bonica's Data (ideal point estimates and campaign finance data)
American National Election Studies (a survey of American adults conducted before and after every Presidential election and after each congressional election)
Charles Stewart's Data Page (miscellaneous information including committee composition, candidate positions derived from the National Political Awareness Test, etc.)
Congress.gov (a cross-linked database on the U.S. congress including bills introduced, amendments submitted, presidential nominations, roll call votes, committee reports, etc.)
Cooperative Congressional Election Study (a survey of American adults conducted before and after recent elections)
David Leip's Election Atlas (historical election data)
Estimates of the Ideological Locations of Bills and Amendments
govtrack.us (data on legislation in the U.S. Congress)
House Committee Request Data
House of Representatives: Office of the Clerk (miscellaneous information including presidential vetoes, discharge petitions, house journals, etc.)
Keith Poole's Data (roll call data for recorded floor votes and various ideal point estimates)
National Annenberg Election Study (a survey of American adults conducted in 2000, 2004, and 2008)
National Political Awareness Test (a survey of elected officials that contains stated positions on a large number of issues)
Survey-based Measure of State Ideology (collected by Erikson, Wright, and McIver and used in Statehouse Democracy)
Congressional Bills Project
Policy Agendas Project
Spending by Congressional District: Berry et al. (2010) Dynes and Huber (2015) Berry et al. (2015) Berry and Fowler (2016)

The State Legislatures
Battista, Peress, and Richman's State Legislator Data (ideal point estimates, committee assignments, and financial interests for state legislators)
Carl Klarner's Dataverse (data on state legislative election returns and governors)
National Conference of State Legislatures (various tables on such topics as term limits, compensation, etc.)
Open States (data on legislation in the state legislatures)
Roll Call Data for the State Legislatures (roll call data collected for a single term for every state legislature)
Survey of State Legislators, 1995 (a survey of state legislators conducted by Carey, Niemi, and Powell)
Survey of State Legislators, 2002 (a survey of state legislators conducted by Carey, Niemi, and Powell)
Shor and McCarty's State Legislative Ideal Point Estimates