Federal Parks
Free entrance days in some United States National Parks http://www.nps.gov/findapark/feefreeparks.htm
National Park Service http://www.nps.gov/index.htm
National Forest Service Main Page http://www.fs.fed.us/
National Forest Service Map of Park Locations http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5274609.html
National Forest Service Free Days http://www.fs.fed.us/news/2011/releases/05/waiver.shtml
Bureau of Land Management BLM http://www.blm.gov/wo/st/en/prog/Recreation.html
Army Corps of Engineers (Lakes) http://corpslakes.usace.army.mil/visitors/visitors.cfm
State Parks
Some state parks are free to visit for the day all year around and those states are Pennsylvania, Missouri, Montana (if you have Montana license plates), Ohio and Tennessee.
Alabama http://www.alapark.com/
Alaska http://dnr.alaska.gov/parks/
Arizona http://azstateparks.com/index.html
Arkansas http://www.arkansasstateparks.com/
California http://www.parks.ca.gov/
Colorado http://www.parks.state.co.us/Pages/HomePage.aspx
Connecticut http://www.ct.gov/dep/cwp/view.asp?a=2716&q=325086&depNav_GID=1650
Delaware http://www.destateparks.com/
Florida http://www.floridastateparks.org/
Georgia http://www.gastateparks.org/
Hawaii http://www.hawaiistateparks.org/
Idaho http://parksandrecreation.idaho.gov/
Illinois http://www.dnr.state.il.us/lands/landmgt/parks/
Indiana http://www.in.gov/dnr/parklake/
Iowa http://www.exploreiowaparks.com/
Kansas http://www.kdwp.state.ks.us/state_parks
Kentucky http://parks.ky.gov/
Louisiana http://www.crt.state.la.us/parks/iparkmap.aspx
Maine http://www.maine.gov/doc/parks/programs/index.html
Maryland http://www.dnr.state.md.us/publiclands/
Massachusetts http://www.mass.gov/dcr/forparks.htm
Michigan http://www.michigan.gov/dnr/0,1607,7-153-10365—,00.html
Minnesota http://www.dnr.state.mn.us/state_parks/index.html
Mississippi http://mississippistateparks.reserveamerica.com/
Missouri http://mostateparks.com/
Montana http://fwp.mt.gov/parks/
Nebraska http://outdoornebraska.ne.gov/parks.asp
Nevada http://parks.nv.gov/
New Hampshire http://www.nhstateparks.org/
New Jersey http://www.state.nj.us/dep/parksandforests/
New Mexico http://www.emnrd.state.nm.us/prd/
New York http://nysparks.state.ny.us/
North Carolina http://www.ncparks.gov/Visit/main.php
North Dakota http://www.parkrec.nd.gov/
Ohio http://www.ohiodnr.com/Default.aspx?alias=www.ohiodnr.com/PARKS
Oklahoma http://www.oklahomaparks.com/
Oregon http://www.oregon.gov/OPRD/PARKS/index.shtml
Pennsylvania http://www.dcnr.state.pa.us/stateparks/
Rhode Island http://www.riparks.com/
South Carolina http://www.southcarolinaparks.com/
South Dakota http://gfp.sd.gov/state-parks/
Tennessee http://www.tn.gov/environment/parks/findapark/
Texas http://www.tpwd.state.tx.us/spdest/
Utah http://stateparks.utah.gov/
Vermont http://www.vtstateparks.com/
Virginia http://www.dcr.virginia.gov/state_parks/
Washington http://www.parks.wa.gov/
West Virginia http://www.wvstateparks.com/
Wisconsin http://dnr.wi.gov/Org/land/parks/
Wyoming http://wyoparks.state.wy.us/