Before the fairly recent introduction of the new top-level domains (.info, .biz, .museum, and so on), there were only seven three-letter top-level domains. Four of them were familiar to any web surfer: .com, .org, .net, and .edu. Two others were less common, but certainly not unfamiliar: .gov and .mil. The mysterious seventh domain, though, was unheard of: .int.
The official top-level domain page describes .int as "Organizations established by international treaties between governments". Odd, then, that my first sighting of a .int domain was tpc.inc, a free fax gateway service. It's established by RFC 1530, but I don't see where an international treaty was involved.
I found that NATO has a .int domain, though other organizations that you would think were formed by international treaties, like the United Nations, don't. Of course, the original justifications for the seven top-level domains have long been loosely interpreted. After all, everyone's got a .com, and even the seemingly unambiguous .edu has been used by the Smithsonian Institution.