I use Unix. Specifically, Linux as OS of preference. The
flavour of Linux that I use is Debian
GNU/Linux. Why GNU/Linux ? Well, Linux is the kernel (the "core",
if you like, of the OS), but many of the tools which are used on top
of the kernel are from GNU.
I've been using Debian since 0.93R6, the first official release.
Since then, I've used at some time or other, most of the newer versions,
and currently possess 3 machines which run Debian. My laptop's running
the most recent version, 2.2. My move to 2.2 has also seen me (finally)
abandon fvwm as my most preferred window manager and investigate
Gnome and, in particular, Enlightenment.
Reports that this list is a resource hog and has memory leaks have thus
far proved unjustified, so I'm happy !
Other Unices I've used include HP-UX (I've also adminned this
a bit), SunOS/Solaris and AIX. I ought to get around to playing with
FreeBSD at some point, but I'm a bit busy right now.
I Don't Do Windows. I cross-compile stuff under Linux, and then
cart the executables somewhere where there's a Windows machine as and
when I need to test the cross-platform availability of my programs. No
machines under my direct control currently have Windows installed on them.
And I'd like it to stay that way !