SilverStorm runs the bulk of its production network and servers on Debian GNU/Linux. It does this for three primary reasons: Stability, upgrade-ability, and community support. These three things combine to create a better ROI than any other Operating System for our business.
SilverStorm designs and manufactures InfiniBand Switches and HCAs enabling 20Gb/s, extremely low latency interconnections for massively parallel supercomputing clusters of commodity machines.
I have tried virtually every distribution of Linux, and always come back to Debian GNU/Linux. It is simply the best.