Sixth floor of the King County Courthouse, in downtown Seattle (516 Third Avenue, 206/296-0940). Very large collection (over 85,000 volumes) including Washington State and federal cases, statutes, regulations, court rules, and practice materials. Several online resources are also available.
The Regional Justice Center, in Kent (401 Fourth Ave. N., 206/205-2900). Smaller, but has a good basic collection.
The U.S. Courts Library is located in the U.S. Courthouse in downtown Seattle (700 Stewart Street, 206/553-4475).
The U.S. Courts Library is open to the public as a courtesy of the judges.
Some Washington State legal materials (such as the Revised Code of Washington and the Washington Administrative Code) and a good collection of U.S. government documents, including the Congressional Record and Congressional hearings and committee reports.
Many other public library branches have some basic legal materials for Washington and the U.S.
County law libraries are often open to the public, as well.