Preview of United States Supreme Court Cases (ABA): This website is the online component to the print periodical Preview of United States Supreme Court Cases, which is published by the American Bar Association's Division for Public Education. In addition to publication highlights and archives, users have access to all U.S. Supreme Court case merit and amicus briefs.
Findlaw: Findlaw's database includes petitioner, respondent, reply, and amicus briefs from 1999-2007.
SCOTUSblog: This blog offers a huge variety of briefs for cases from 2007-present.
Office of the Solicitor General: The Solicitor General's office provides full-text copies of all briefs filed by the Solicitor General, excluding IFP (in forma pauperis) cases. The briefs are organized by the Supreme Court term in which they were filed (which is not necessarily the same term in which the case was heard). From 1998-present, all briefs are fully searchable by keyword, type of filing, or client/subject. From 1982-1998, only merit briefs and responses are provided; only select briefs are searchable, and those by keyword only. The other briefs from this period are organized chronologically.
Other federal agencies: Federal agencies often provide the text of party or amicus briefs they have files in high-profile cases. If you are looking for a brief filed by a specific agency, try searching that agency's website.
Westlaw:Westlaw's coverage includes petitioner, respondent, and reply briefs, as well as selected joint appendices, supplemental briefs, and supplemental appendices from 1990-present, petitioner respondent, appellant, and appellee briefs from 1993-present, and amicus briefs from 1995-present.
Lexis: Lexis includes all merits briefs from 1993-present, but no joint appendices. Appendices to individual briefs are included. Merits briefs and joint appendices are available for the terms from 1979-1992.