This is expanding on my answer to David Thoreau's blog comment. (The picture is of where I'm staying tonight, I'll blog about it soon.)
There are many places you can camp for free in the western United States. While the 14-day limit is very common, what you need to do to reset the counter varies. In some places, like the Kofa National Wildlife Refuge, it's 14 days per year for the entire Refuge. On the 14-day BLM camping areas near Quartzsite, it's move at least 25 miles for 14 days before you can return. Some places the rules are posted, others you need to research.
Most BLM land that is not otherwise restricted allows 14-day camping. (The rules vary somewhat by BLM office.) Some areas are "open" where you can drive or camp anywhere, others are "limited" where you can only drive on established roads and park/camp near them, others are "closed" where no driving is allowed. In general, if there are BLM signs or it is marked BLM (and not wilderness or ACEC) on a map, you can assume limited unless otherwise posted. BLM roads tend to be unmaintained dirt roads, even where they are signed. Some places disallow camping but allow other uses. The BLM has several LTVAs (long term visitors areas) near the Colorado River, where you pay a $180 fee for October through April, and can stay at any of them for the entire season. Some have dump stations, trash bins, water, and porta-potties. Alternatively, you can pay $40 to stay at an LTVA for two weeks. In some areas, BLM has campgrounds which may charge. There are other BLM fee areas, but I have not found out the rules for them. BLM offices have maps for sale as well as free handouts, and most of their employees are helpful.
In many National Forests, you can distributed camp for free. The rules vary, you need to check the forest you are interested in. The 14-day limit may be per ranger district or forest, so you may be able to move a few miles down the road and stay another 14 days. Some National Forest campgrounds are free, but most charge. Frequently they are maintained by a contractor, I'll be working in one this summer. A few forests charge to park, the $80/year America the Beautiful pass covers this.
Most areas operated by the National Park Service do not allow camping other than in designated campgrounds that charge for the privilege. (There are some exceptions.) The America the Beautiful pass covers then entry fee to the park but not camping.
The (Army) Core of Engineers (CoE) operates a number of campgrounds near dams they have built. They do not normally allow distributed camping.
Some retail businesses allow overnight parking, WalMart in many places allows this but I've found it disallowed in most of the places I want a place to stay overnight. There is a yahoo group walmartrving that has a listing in their files section. Some Casinos allow overnight parking, see Casino Camper.