LinkExchange
LinkExchange was one of the Late-1990s and early 2000s largest FreeBSD users in the world, only surpassed by Yahoo and Hotmail.
One of several early Ad Networks, LinkExchange served billions of ads per day using 5 FreeBSD Boxes running 2.2.5 and eventually 3.x prior to being acquired by Microsoft.
Several internet luminaries emerged from it - Alfred Lin (Sequoia), Scott Bannister (Ironport/Paypal), Max Levchin (Paypal) and Tony Hseih (Zappos/Venture Frogs).