HRMINI

National parks offers second fee-free weekend
If you missed out on it the first time around the National Park Service’s second fee-free weekend is this weekend starting Saturday. Choose between more than 100 national parks to visit this weekend and get the usual $10 per adult entrance fee waived.
The first fee-free weekend took place June 20-21. Your last chance to take advantage of this offer will be Aug. 15-16. The fee waiver not only includes entrance fees, but also commercial tour fees and transportation entrance fees.
The waiver does not cover other fees such as camping reservations, tours and concessions.
In Virginia there are 10 parks that will waive entrance fees, including Colonial National Historical Park (Historic Jamestowne, Yorktown Battlefield, Colonial Parkway and Cape Henry Memorial). 
Great Falls Park in Mclean, Petersburg National Battlefield and Shenandoah National Park will also waive entrance fees on Saturday and Sunday.
For free, nationally travelers can also visit Yellowstone National Park in Wyoming, Montana and Idaho, Yosemite National Park in California and the Badlands National Park in South Dakota

Maryland
Antietam National Battlefield
Assateague Island National Seashore
Chesapeake and Ohio Canal National Historical Park
Fort McHenry NM and Historic Shrine National Monument
Fort Washington Park
Harpers Ferry National Historical Park

North Carolina
Wright Brothers National Memorial

Virginia
Appomattox Court House National Historical Park
National Memorial
Assateague Island National Seashore
Colonial National Historical Park
George Washington Birthplace National Monument
George Washington Memorial Parkway's Great Falls Park
Manassas National Battlefield Park
Petersburg National Battlefield
Prince William Forest Park
Shenandoah National Park

More parks listed here:

http://weblogs.dailypress.com/features/shoppingblog/2009/07/national_parks_offers_second_fee-free_weekend.html

http://www.nps.gov/FINDAPARK/FEEFREEPARKS.HTM