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For myself,
Windows 10 Preview in a VM (for gaming without booting my regular Windows)
Windows 8.1 on my laptop
Arch Linux on my laptop (my primary OS)
Linux Mint on my laptop (I keep it around mainly for Steam, which I have not succeeded in running on Arch)
ReactOS in a VM (it's an open-source Windows clone)
Chrome OS on my Chromebook
(Oh, dammit, the poll option isn't working on Chrome and editing it seems to have removed the poll... I'll put it back in when I next have access to something that's not a Chromebook. Speaking of which, anyone know what's going on there? [Avatars not loading; the create-a-poll option not showing up; the unhide spoiler button not doing anything]. Or am I the only one? [Chrome on Chrome OS, Firefox and Chromium on Arch Linux; works fine with Project Spartan on Win10 preview])