It was linux, but I also ran OS X on a PC for a while, had my gaming rig dual booting XP Pro and Mac OS X. Might do it again too, but dont have any room in the case for another drive, video card is blocking atleast 2 slots for a drive, the cables have no room to plug in, I need a new case, just got this one too, had spots for 3 fans, got some good fans for it, but there are just too many things about it that are starting to piss me off.
Also dont have anymore IDE spots open, and dont have any spare SATA drives to shove in.
What id love to do is tri-boot, Windows XP Pro, Linux, and OS X.