Oktet Labs, Saint Petersburg, Russia
We use Debian on all of our servers and on the vast majority of our workstations, which are between 50 - 100 installations, including virtual hosts.
Our development servers are used both for embedded software development and for intensive testing of our and our customers products. Our infrastructure servers run non-vanilla versions of Apache with in-house patches, as well wiki, mail, VCS services, and etc...
The main reasons for choosing Debian were the simplicity of administration and stable releases.