African Lottery, Cape Town, South Africa
We are using Debian for advanced cryptographic programs like ISAAC, and for random number generators (RNG). What's more, it's used for simpler tasks like apache web server, mysql database, and ftp. It's running on one 64 bit Xeon(R) CPU E5-2620 station.
Debian was chosen because of no vulnerabilities (malware & hacks), and great support.