We're a bike messenger collective based in Berlin. In our office all boxes are running Debian, which means that there are two workstations and one server. All are running Debian Squeeze with the latest updates.
The workstations are using Gnome as the desktop environment, and some extra software like taxbird. Recently I purged OpenOffice and installed LibreOffice via the backports-system. The PCs are used for the normal, every-day office work.
The server is running munin, sshd, fetchmail/procmail/dovecot/postfix, lighttpd, some security tools and samba for serving files over the network. Also I'm using mdadm to run the two harddisks in raid1. Each night duplicity is backupping all the important files to a server at an isp.
I've choosen Debian because I've been a fan of the OS for some years. It's performing quite well, once set up it just works and, last but not least, it's FREE. I think my colleagues are also quite happy with this system.