We use a combination of Debian servers and Ubuntu workstations to build or enhance networks for ourselves and several other small-medium sized businesses and organizations, often in cross-platform environments. Our Debian servers mostly run software like Samba, Request-Tracker, Ledgersmb, and Mediawiki, but we also have hosting services where we use Apache2 and Qmail, along with various supporting software. Another common use we use Debian for is firewall and bandwidth management solutions, using software like iptables/xtables, Openswan, and Squid/Dansguardian.
The preference for Debian originates from apt, many years ago. Continued devotion stems from excellent documentation and adherence to open standards, both of which have served very well to prevent the need to look elsewhere.