Mac OSX Mavericks. Used Windows 8 alongside it for awhile, but my copy of Parallels Desktop became outdated and I couldn’t use it anymore. Didn’t bother to upgrade since I realized it had outlived its usefulness.
For the main PC, I use windows 7 ultimate, for the laptop I use gentoo linux for servers, I also have an andriod 4.4.2 Galaxy S5 phone.
I had Gentoo on the main comp too for a while, but the drive started to die so I went back to the other drive that had my old win 7 install on it, I still have a disk image of that Gentoo os, so I could restore it if I got another drive sometime in the future.
I had, actually.
Between 11.04 and 11.10, I tried out a few others that way and ended up choosing XFCE. Almost went with LXDE, but I couldn’t fully give up compositing.
When my yearly fresh-install came around, Xubuntu was simply the more efficient setup route.
Also, since people are mentioning their mobile devices as well:
LG Nexus 5 w/ CM11 (M9)
On my desktop, Windows 8.1 Pro 64-bit, and on my Dell Venue 8 Pro, Windows 8.1 32-bit, on my phone, Android 4.4.2. Eagerly awaiting both Threshold and Android L.
Main OS: Mac OS X 10.9 (I have a hackintosh, switching to 10.10 when it’s available. )
Server OS: Debian 7.x or Ubuntu 12/14 depending on the situation, for minecraft I prefer Ubuntu, and for everything else… Debian. <3
Mobile OS: iOS 7/8