NOC, RG3.Net, Brazil
We use Debian mainly on two of our servers (of a total of four servers). One of them is a web cache running Squid, serving more than 600 users (we run a WISP) and the other one is running ZoneMinder, recording all our cameras activities. Also, we use Debian in two workstations of a total of three.
We chose Debian because of its stability and simplicity. We think that many other distributions are bloated. Also, we like the way Debian chooses which packages will become part of the stable distribution.