Mac OS 8.6. At my mom's old job, that's what they used for computers. eMacs with OS 8.6. I actually have one of those sitting around somewhere. I should dig it out and use it. So awesomesauce.
I enjoy Win7 so far, haven't really tinkered enough to delve into the whole Linux scene. On my to-do list though!
I believe that I made the thread you are talking about: http://www.everypony.com/forums/showthread.php?4039-What-OS-does-everypony-use Whether the old thread gets re-used, merged or abandoned, I don't really care. Anyway, I'm using Linux Mint currently and it's great. I prefer it to Windows 7 but it doesn't have the same compatibility as Windows. And since I'm relying on certain programs (Microsoft Office), Windows would usually be my best bet.
Or rather: It's easier to give it functionality to do different things. But first you have to give it that functionality. And all of the relevant dependencies.