We use Debian on our servers for intranet infrastructure (dns, dhcp, firewalls, ...), as web applications server with apache, php, mysql and postgresql/postgis. We also use motion for web cams. Furthermore we use Debian to develop veterinary and biology applications. On desktops we use Quantum-Gis and R to study the animal movements we get from the PPT's from Argos. We also use libreoffice, gimp, scribus, iceweasel, icedove, etc. Debian is installed on 5 servers and on 5 workstations.
We use Debian because of its quality. Also because it is free, but quality is more important to us.