Passport America, Good Sam, and KOA. As others have mentioned, the Passport America rules vary according to the individual campground. The 50% discount is typically (but not always) during the less busy times when campgrounds can not sell all their sites at full price.
Good Sam is okay, but in many places, the 10% discount does not even cover the sales tax. We have also found that some campgrounds inflate their price only to then discount it. So sometimes the 10% Good Sam rate is not really any savings. The 50% Passport America discount is sometimes off a make believe price that no one ever actually pays. Many times the discounts are legit, but some campgrounds play games.
The KOA card can be a good deal if you like their campgrounds. Last year they ran a one day special for their Value Card for $5 around Thanksgiving.
I tried a Thousand Trail Zone Pass for a year. They were okay, but the places we visited had sold their nicer sites to people who kept their RVs permanently parked, so the sites were unavailable for use. The parks also seemed less well maintained. With the Zone Pass you can cancel (30 days written notice required) and not be locked into a long term commitment. They often have deals on the Zone Pass at a discount if you want to try it. Just read all the fine print.
2010 Georgetown 373
2013 Jeep Wrangler