The country code domain for Antarctica.
The fact that Antarctica isn't actually a country didn't stop it from getting a country code domain, since they are assigned based on the ISO country code list, which has assigned "AQ" to Antarctica. Thus, Antarctica is one of several non-countries that have country code domains, along with the Palestinian Authority (.ps), the former Soviet Union (still hanging onto .su), and various territories and possessions of other countries that managed to get into the ISO list separately, like Puerto Rico (.pr). The upcoming .eu domain for the European Union will join this group.
I haven't been able to find information on how to register a .aq domain or what the eligibility requirements are (Antarctica has no citizens or permanent residents, but has various scientific bases run by several different countries). However, there are several active Web sites in .aq domains, including esouth.aq and scott.aq.
You can find sites in a specific domain by going to Google, going to its Advanced Search page, and typing the domain you want in the "Domain" field, and typing random garbage in the "Find results... WITHOUT the words" field (necessary because Google requires something to be filled in either the "with" or "without" fields -- pick a string unlikely to turn up in any site).