Mystery of roman numerals

Two important events, the Super Bowl (Super Bowl XVLI) and the London Summer Olympiad (XXX) both use roman numerals as identity.

The only other thing I can think of that is well known that uses roman numerals for identification is the pope.

But the most curious thing is that the number 666, when written, uses each roman numeral there is and uses it only once: DCLXVI.