Govinfo is the Government Publishing Office's main document platform. Use it to find materials and reports from across the federal government.
The Catalog of U.S. Government Publications (CGP) is the finding tool for electronic and print publications from the legislative, executive, and judicial branches of the U.S. government. Includes records from 1976-present, though it will grow to include records for publications dating back to the late 1800s.
USA.gov is the U.S. government's official Internet portal.
HathiTrust (pronounced 'hah-tee') is a partnership of libraries and research institutions that have come together to build and share a digital repository of millions of print works. HathiTrust has a wealth of scanned U.S. federal government documents available. U.S. federal government documents are generally freely viewable by anyone, but only Pitt affiliates can download entire items.
HathiTrust has a U.S. Federal Document Collection. Keep in mind that U.S. federal government documents may be available on HathiTrust but not present in this collection. If you're searching for a specific document, using the general HathiTrust search is the best starting point.