We use Debian on all of our company workstations and servers (about a dozen of each), and on 99 procent of the servers we deploy and support for our customers (hundreds). We primarily use Perl, PostgreSQL and MySQL, Apache/mod_perl, HTML::Mason, and LaTeX. The company founder has been a Debian developer since 2001.
We use Debian because of the rock-solid stable releases, painless remote upgrades and security support for oldstable for a full year. Debian's packaging of Perl CPAN libraries is vast and second-to-none. The PostgreSQL upgrade path that decouples database upgrade from OS upgrade is invaluable. We appreciate that Debian is not developed by a single company.