Study of the U.S. Congress is one of the most quantitative areas in political science, and a wide variety of datasets exist.
Roll Call Voting Data
Since Roll Call voting is so central in Congress (that is, votes cast on the floor of the House or Senate) There are a lot of available datasets on this.
Charles Stewart's Congressional Data Page (Congressional Committees, Congressional Election Candidates, Voting Scores)
Election Data (Citizens Voting in Congressional Races)
CCES - Cooperative Congressional Election Study
Congressional Bills Project This is a compendium/link site covering a wide variety of congressional data sets.
Clerk of the House