National lands admission-free days

All national parks, national wildlife refuges, and many areas managed by the Bureau of Land Management will offer free admission on June 5-6, August 14-15, September 25 (Public Lands Day), and November 11 (Veterans Day).

National parks at http://www.nps.gov/findapark/feefreeparks.htm
National wildlife refuges at http://www.fws.gov/refuges/
Bureau of Land Management areas at http://www.blm.gov/wo/st/en/prog/Recreation/BLM_Fee_Free_Days.html