At my office, I use a combination of Windows 2000 Pro/Server/Adv. Server, Windows NT Server, Windows 98SE/98/95, and even one Novell Netware box.
Home box #1 (Optimus):
Linux Slackware 8.1
Windows 98 Second Edition
Home box #2 (Perceptor):
Linux Slackware 8.1
Home box #3 (Ultra Magnus):
It was Linux Slackware 8.1 and will be Linux again when I get the box back, but its on temporary loan to someone else running Windows XP Pro.
Home box #4 (Grimlock):
Linux Slackware 8.1
Home box #5 (Bumblebee):
Linux Slackware 8.1 (will be OpenBSD, then probably Net/FreeBSD once I get the urge to slap 'em onto the box).
Home box #6 (Ratchet):
(You guessed it) Linux Slackware 8.1