High Energy Physics Group, Institute for Theoretical Physics, University of Technology, Vienna, Austria
Except for one heretic (who uses Slackware) we all use Debian ever since said heretic introduced it to the institute in the mid-1990s (he switched later, God knows why...). We have 5 servers, 29 workstations, and 3 notebooks with Debian.
The software is mostly scientifically relevant stuff like LaTeX, C++, some graphic tools like Xfig, standard Apache and e-mail/Internet stuff and things that help to scan and burn.
Why we run Debian: very convenient upgrading mechanism, security issues, rarely need the latest drivers for fancy hardware anyway (so no drawback in using Debian), installation via net (local mirror) straightforward - well, and of course additionally all the standard reasons why Linux over Windows, but that is not Debian specific.