Microsoft use a BSD to power many of their web services. I'm not sure which one, but there are a few such as OpenBSD, NetBSD, FreeBSD, Darwin, OpenDarwin, Dragonfly, etc. (They probably use OpenBSD).
They are all incredibly fast and stable operating systems. Far better than Linux or Unixware, but the BSDs are less famous because a long court battle (against AT&T) made the media think Linux was a safer bet than BSD.