https://www.supremecourt.gov/ (new website 7/31/17 to reduce "linkrot" and provide more detailed court calendaring)
U.S. Reports aka United States Reports (Library of Congress digital collection; 35,000+ official reports of decisions for the Supreme Court, 1791-present): https://www.loc.gov/collections/united-states-reports/ (about)
http://www.supremecourtus.gov/
United States Reports: https://www.loc.gov/collections/united-states-reports/ (bound case reporters that are the official reports of decisions for the United States Supreme Court. You can translate a citation from a nominative reporter to a volume of the U.S. Reports by using this chart.
Opinions: http://www.supremecourt.gov/opinions/opinions.aspx
See also Justia: https://supreme.justia.com/