• washingtonpost.com's U.S. Congress Votes Database (102nd Congress [1991]-present)
  See votes on particular bills, all votes for a particular member of Congress, votes with wide or narrow margins, late-night votes, most-voted-on bills, vote-missers, and more. Very current. Allows users to set up RSS feeds.
• Office of the Clerk, House of Representatives (101st Congress, 2nd session [1990]-present)
• Senate Votes (101st Congress, 1st session [1989]-present)
• GovTrack.us (106th Congress [1999]-present)
  Includes other information about legislation and members of Congress, including campaign finance information. Allows users to set up "monitors" for online tracking and notification of activity for every person, bill, subject term, or committee in Congress.
• LexisNexis Congressional (100th Congress, 2nd session [1988]-present)
  Choose Legislative Histories, Bills & Laws; Keyword Search; select Floor Votes radio button

• Roll call votes are recorded in Congressional Record, available online from the following sources (dates of coverage vary):
• HeinOnline
• GPO Access
• LexisNexis Congressional
• A Century of Lawmaking (Library of Congress)
.

• Congressional Record volumes from 1873-1994 are also available in print at the Social Science Library.

• "Key votes" (i.e. selected high-profile votes) are available from CQ Press Congress and the Nation online edition, back to the 79th Congress (1945).
• Roll call data from the 1st Congress to present are freely available for download from Voteview.com. These data are meant to be opened and analyzed in a statistical software package.
• OpenCongress

Voting in Congress (LLRX.com) is a brief article explaining some of the nuances of Congressional votes.