What OS do you use?

[Deak]

Windows 7 on laptop
iOS 5 on iPhone

[AgnesIO]

Windows 8 Dev Preview
Windows 7
Android 2.3.4
Webos

[Oleh]

Windows 7. due to the programs I use on a day to day basis not having primary support for other OS's.

[xdinosaurdaniel]

Windows 7 usually

[Mark]

Windows 7 on Laptop
iOS5 on iPhone

[Chippiewill]

Windows 7, because it was my idea.

And Android on Phone, because I like google.

[xxMATTGxx]

I use the following:

Windows 7, because it was my idea also.
Windows Phone 7 on my mobile device

[dirrty]

ios 5
os x lion

[Recursion]

In order of use on each system...

Desktop: Windows 7 Pro
Work Laptop: Windows 7 Pro, Windows 7 Enterprise and Ubuntu Linux
MacBook: Windows 7 Pro, OS X Lion
HTC Desire: CyanogenMod 7.1 Android 2.3.7
iPad: iOS 5

Various VMs running FreeBSD, Server 2003R2/2008R2, Ubuntu Server

[Markeh]

Desktop: Win 7 Ultimate 32-bit (bought by mistake, never bothered to change)
Main laptop (HP): Win 7 HP 32-bit
Other laptop (Toshiba): Win 7 Ultimate 64-bit
Workshop PC (Lenovo): Win 7 Ultimate 32-bit
First phone (HTC): Froyo
Second phone (BB) OS5 I think (it's not here yet)
iPod: 4.0 (soon to be 5 when I buy a new iPod)

All my parents PCs (a P4 Packard Bell, an old ThinkPad X31, and a P4 Fujitsu laptop) are on XP (the Fujitsu's going in favour of an Eee PC soon)