PACER (Public Access to Court Electronic Records) is an electronic public access service that allows users to obtain case and docket information from Federal Appellate, District and Bankruptcy courts. Creating an account in PACER is free, as is viewing judicial opinions, but accessing other court filings incurs a $0.10/page fee. However, users who incur less than $30 in charges in any quarterly cycle will not be billed. Coverage varies by jurisdiction.
The Caselaw Access Project offers free, public access to over 6.5 million decisions published by state and federal courts throughout U.S. history and covers 360 years of US Legal History. The Caselaw Access Project also offers free tools to analyze court opinions through their Caselaw Access Project API.
Court Listener is a free website that gathers judicial opinions and related materials throughout the U.S. It does not provide access to statutes, constitutional provisions, or regulations. It offers visualizations of U.S. Supreme Court decisions, which help identify important cases related to your research.