Forms provided by the Administrative Office of the U.S. Courts, which can be used in all federal courts. Forms are organized into different categories such as Attorney, Bankruptcy, Civil, Criminal, Jury, etc.
List of court websites including to the Supreme Court, Federal Courts of Appeal, District Courts, Bankruptcy Courts, and other national organizations, which have their own form libraries.
Official USCIS forms with details and instructions for filing. Some forms may be filed online.
Search results of the US website hub for "forms." Returns many different agencies' webpages featuring their own forms.