There are many unexpected places to camp for free. In the western part of the USA there are thousands of places in National Forests and BLM land. In the rest of the USA it is harder to find places. Plum Creek, a lumber/forest company allows 14 days.
It is often hard in the southeast, where Plum Creek has lots of acres. Check out this link to find out more.
Then there are the truck stops, which offer a night next to big rigs and showers and food and gas. Check out this link, which gives a way to download the application for your phone.
(Can we really still call those things phones?)