FreeBSD
Used by 12 peoplefreebsd.org ↗
“I switched to FreeBSD, which I still love, but missed having Flash for YouTube.”
Derek Sivers Interview, TED speaker, founder of CD Baby · via Uses This 
“I also have a server running FreeBSD on the east coast, but that's going away soon.”
Paul Graham Interview, Head of YCombinator, creator of the Arc language · via Uses This
