An $80 Interagency Annual Pass covers entry to US National Parks AND National Forests.
Otherwise you would need to pay on entry, or maybe by a local pass, like the $30 Adventure Pass for Southern California National Forests.
I like US National Forest sites. The Forests are less developed than the National Parks. But they are a lot bigger and much less crowded. Forest camp sites are generally free (maybe always, I have never seen a fee.) The National Forests don’t require a pass for entry like the Parks, but you need one in your vehicle when its parked at the camp grounds.
You can generally simply camp on National Forest land, too. You are not restricted to the developed camp grounds.
US Forest Service
http://www.fs.fed.us/
Map of US Forests
http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5274609.html