We use Debian GNU/Linux to fill various roles within our organization. Debian GNU/Linux powers our developer server. The system runs trac and subversion very well. Both applications as well as system logins are configured to authenticate against an Active Directory Domain.
Debian also powers our web application hosting environment. We currently have 5 systems running JBoss and 5 systems running MySQL. We also have a Debian powered load balancer running pen.
The main reason for choosing Debian over other distributions is the package management, number of available packages, ease-of-use, stability, amount of quality documentation, and cost.