Boeing Commercial Airplanes Flight Test Data Systems, Unites States
Debian is used on very specialized equipment that is loaded on planes to gather testing data. There is other uses of Debian also. Most of the installations I have encountered run Debian 3.1. About 30 machines that I know of.
Debian was chosen due to support of Power PC chips and the free nature of Debian. Debian was not really chosen over the competition, as other platforms are in use also, including Red Hat, Yellow Dog, CentOS, Fedora, Ubuntu.