Washington and Lee University
The official site of the Court now offers recent slip opinions, transcripts of oral arguments, and the texts of U.S. Reports (which, by the way, is considered the only official source for opinions).
This service from the U.S. Government Printing Office provides links to the above site, as well as to opinions/decisions from 1937-1975 and 1992-2000.
This U.S. Library of Congress site collects documents relating to individuals nominated to the Supreme Court, including law journal articles, argument transcripts, Congressional hearings, and more. Best coverage is from 1990's-to-date.
U.S. Reports, opinions, and other materials. (W&L subscription).
"Earliest opinion dated from January 1790 through current," plus additional material.
For off-campus access, go to LexisNexis Academic -- Legal and select "Federal & State Cases" from the column on the left, then "U.S. Supreme Court Cases, Lawyer's Edition" from the Sources drop-down menu. (W&L subscription)
Recent decisions (1990- ), plus historical material. (Cornell University Law School.)
Searchable opinions (1893- ), provided by the commercial FindLaw service. (Some sections of the site might require registration.)
Multifaceted (and multimedia) site even includes audio of oral arguments for an ever-growing number of cases, as well as written opinions, summaries, etc. (Northwestern University.)
Extensive educational materials on a handful of important cases, beginning with Marbury v. Madison. From the Supreme Court Historical Society and Streetlaw.org.
(4th ed.) Congressional Quarterly. Available online.
(5th ed.) CQ Press. Available online.
Macmillan. Available online.
(2nd ed.) Oxford University Press. Available online.
(2nd ed.) Oxford University Press. Available online.
Congressional Quarterly. Available online.
Oxford University Press. Available online.
(15th ed.) Rowman and Littlefield. Available online.