Essential Systems, UK
We use Debian on two fronts: The first is in our 14 web application servers (Dev and Live). So everything from websites to our CRM/ERP system. The second area we use it is on our Asterisk PBX servers which we've currently got four of across the country all managing 50-200 peers each over VoIP and ISDN.
Debian's been great for the company - reliable, clean, simple and it just works!