Aside from parts of Antarctica, there is only one section of land on Earth that isn't claimed by at least one nation. It is called Bi'r Tawīl, and it lies between Egypt and Sudan. Neither claims it, because there is a disputed area nearby, and claiming Bi'r Tawīl would invalidate that claim. I think one of these days, someone is going to drive through Egypt and raise their own flag there.
Read more here:http://en.wikipedia.org/wiki/Bir_Tawil