With close to 400,000 systems, the Naval Marine Corp Intranet (NMCI), managed by EDS/HP, is "the largest internal computer network in the world" and has been using Diskeeper on their systems for close to a decade now: 

"We run Diskeeper on our two production web servers and the development server that we are using to host a web-based warfare systems interface management tool. The name of the application is Warfare Interface Systems Engineering (WISE). 

I am a big fan of Diskeeper because I am all too aware of the problems that  fragmentation can cause on a system. I use it at home as well. I specifically like the feature that keeps your drives defragmented automatically via a background process that does not rely on manual defragmentation of the drives. Because I know that if you leave that up to a user or even some administrators to do, it will never get done and people will be wondering why their system performance is starting to degrade.  

Our office systems are a part of the Navy/Marine Corps Intranet and all of these systems have Diskeeper running on them as well [almost 400,000]. I know all too well this group of users in our command. I used to work IT support and they would never defrag their systems. I do some PC work on the side with home users and I see the same thing there."

Mike M, US NavyNSWC Dam Neck

Virginia Beach, VA