I believe that the US Air Force academy (and surrounding base, which is quite large) has it’s own zip with several +4 variations.
My former employer (Parsons, in Pasadena, CA) had its own zip code (91124)
My alma mater (Cal Poly, San Luis Obispo) had its own 5-digit zip, I think. I don’t remember what it was though.
From Wikipedia
[quote]
Some places with extremely large volumes of mail have their own full ZIP code, including 81009 for the the Federal Citizen Information Center (FCIC) of the U.S. General Services Administration (GSA) [1] in Pueblo, Colorado, and 10048 for the World Trade Center complex in New York, New York. The White House has its own secret ZIP code, separate from the regularly-occurring ZIP code, for the U.S. President and his family to receive private mail.
I’m pretty sure the Empire State Building has it’s own ZIP, too.
From the USPS, here is a partial list of unique ZIP codes from New York.
They seem to be pretty common.