TLD .int
The top-level domain .int was the first permanent addition to the generic (non-country-code) top-level domains after the original group of domains introduced in 1985. (A .nato domain was added earlier, but went unused as NATO chose to use a .int domain instead, and was eventually removed.)
The .int domain is intended for international treaty organizations as designated by the United Nations. It is administered directly by IANA (instead of being delegated to outside registries and registrars as are other domains), and has among the tightest enforcement of eligibility criteria of any domain.
Despite this tightness, a few organizations that would not meet the current criteria managed to get .int domains in its early days, and are grandfathered in. For instance, the YMCA has a .int domain, which is presumably fun to stay at.
