This holiday weekend marks another fee-free time to check out one of the participating parks. Olympic National Park has a news release about their plans. Participating parks in Washington State include:

Fort Vancouver National Historic Site
Lewis & Clark National Historical Park
Mount Rainier National Park
Olympic National Park
Whitman Mission National Historic Site

and in Oregon:

Crater Lake National Park
Lewis & Clark National Historical Park

The nationwide list has all the participating parks, and more information on the Free-Entrance program can be found here.