SRON: Netherlands Institute for Space Research
SRON uses Linux extensively throughout the organization, running web servers, file servers, and (scientific-) desktops.
We've consolidated on Debian GNU/Linux for its stability, ease of administration, slow release cycle and overall software compatibility. Apart from the technical reasoning, there is also the political and structural: Debian is non-profit and non-commercial. It cannot be bought or sold and therefore we expect it to stay around for a long time to come.
We have +/- 130 physical Debian Linux workstations and 20 virtual desktops always running Debian stable. In addition, we have around 50 servers, both physical and virtual running Debian stable, used for file-storage, and GRID computing.