Game Development Club, Worcester Polytechnic Institute, Worcester MA, USA
Debian currently runs on our main webserver (lumi.gdc.wpi.edu — Dual AMD Opteron 4180), which also hosts our game servers. It is planned to be installed on our other server (poe.gdc.wpi.edu — Dual AMD Opteron 240) soon when it is retooled to serve new needs in our organization as Debian has served us extremely well so far.
We chose Debian when building our newest server (lumi.gdc.wpi.edu) due to its ease of setup, its stability, and for the administrator's familiarity with it as a long-time Debian user. The excellent APT system, and Debian's security were also factors in the decision to use Debian on our machines.