Windows 7 Pro x64 is my main, but I dual boot with Ubuntu 9.10 (I'm still new to all this linux stuff, so I went with Ubuntu as I could install it through Wubi)
I am enjoying the dipping into Ubuntu, it's a nice OS, but it wont replace Windows 7 for me, as Windows 7 is just too awesome. But who knows what could happen in the future!
And seeing as people are giving system specs too now;
Intel Core 2 Quad Q6600 @ 2.4GHz
4GB OCZ DDR2 ram
ATI Radeon HD4850
hard drives of various sizes adding up to about 2TB
Not as cutting edge as it was when I built it in July 2008, but it still plays everything I want it to.
Got a laptop too, an Asus R1F Convertible Tablet, which is currently running Windows 7 Pro X86
That's got a Core 2 Duo 1.8GHz
2.5GB DDR 2 ram
GMA 950 graphics (games work so well on it! Half-Life 2 at 20fps - AMAZING!)
100GB hard drive
and a completely knackered battery (why are laptop batteries so shit?)
Sod it, phone too - an ageing Nokia N95-8GB soon to be replaced by a Nokia N900 when my upgrade period comes around at the beginning of April
hooray at last!