From the always excellent Strange Maps:

The Bir Tawil Triangle is a desert of sand and rocks on the border between Egypt and the Sudan. It is also officially the most undesired territory in the world. Bir Tawil is the only piece of land on Earth (*) that is not claimed by any country – least of all by its neighbours. For either of them to claim the Bir Tawil Triangle would be to relinquish their claim to the Hala’ib Triangle. And while Hala’ib is also mainly rock and sand, it is not only ten times larger than Bir Tawil, but also adjacent to the Red Sea - so rather more interesting.

[Thanks, Stefan]