2009 Post: The National Parks are going to free admission for three weekends this summer. On the weekends of June 20-21, July 18-19, and August 15-16, all 147 parks that charge an entrance fee will eliminate them. This includes even the big ones like the Grand Canyon, Yellowstone and Yosemite. You'll save anywhere from $3 to $25 on the entrance fees which gives you more money to spend in local shops.
It's important to note that regular fees will still apply for camping, tours, concessions or other fees.
Will they all be packed on those weekends? Probably but who knows for sure. I would count on the most popular parks being pretty crowded so it's up to you to save money and get close to others or just pay and enjoy it more on another weekend.
This summer is an odd mix of people traveling less and the fact that visits to National Parks are cheaper than many other kinds of vacations. Most people live within less than a days drive to a National Park of some kind. You can view our handy map of National Parks with camping here. There are other National Parks that do not have camping as well.