FindLaw offers a database of case law from the U.S. Supreme Court and U.S. Circuit Courts of Appeal, as well as several state supreme courts.
U.S. Supreme Court Opinions – 1791 to the present
U.S. Supreme Court Center External - includes Court opinions and a blog regarding recent Supreme Court events
U.S. Federal Appellate Court Opinions – 1995 to the present (may be expanded, based on the circuit)
U.S. State Supreme, Appellate and Trial Court Opinions – 1997 to the present (may differ, based on the jurisdiction)