Whatever gets the job done.
Windows 7 dual booted along with Mint Linux on my primary personal laptop
Windows 8.1 on my travelling personal laptop
Windows 7 on my general purpose (e.g. print) server
Windows 7 on my gaming desktop
FreeBSD / FreeNAS 9.2.1.5 on my NAS / media / cloud server
Some flavor of embedded Linux on a bunch of things (wireless routers, Roku, etc).
Ubuntu 14.04 on three virtual private servers (2 in North America, 1 in Europe)
Windows 7 + Mint Linux in a VM on my work laptop.
Various versions of Red Hat Linux and derivatives on more systems than I can remember at work (the oldest is RHEL ES3, probably 12+ years old)
Even have an HPUX B.11.00 dinosaur that's we still use at work - I believe that one's been in use since 2000.
Windows 7 dual booted along with Mint Linux on my primary personal laptop
Windows 8.1 on my travelling personal laptop
Windows 7 on my general purpose (e.g. print) server
Windows 7 on my gaming desktop
FreeBSD / FreeNAS 9.2.1.5 on my NAS / media / cloud server
Some flavor of embedded Linux on a bunch of things (wireless routers, Roku, etc).
Ubuntu 14.04 on three virtual private servers (2 in North America, 1 in Europe)
Windows 7 + Mint Linux in a VM on my work laptop.
Various versions of Red Hat Linux and derivatives on more systems than I can remember at work (the oldest is RHEL ES3, probably 12+ years old)
Even have an HPUX B.11.00 dinosaur that's we still use at work - I believe that one's been in use since 2000.