Single Spot Camping is a new Swedish website that connects people with land to campers who want a place to throw down a tent. It's similar to Couch Surfing, a site that connects people with couches to people who want to sleep on them while traveling.
Right now Single Spot covers Sweden, Norway, Denmark, Finland, Australia, and the US, though the fact that the site is in Swedish makes it hard to see it taking off here. I wouldn't be surprised to see something like this pop up stateside sometime in the next year or two. It's too good of an idea not to.