Microsoft's Ultimate Beta Tester? Itself:

• 60,000 machines were running Windows XP Service Pack 2 during its beta phase
• By mid-June
  • Microsoft had 16,000 Windows Vista Beta 2 users (I'm one of them)
  • 25,000 on Office 2007 Beta 2 (I'm one of them)
• 6,000 mailboxes are now on Exchange 12 (I have no idea if I am one of them)
• 30 Longhorn Servers are already in deployment
• 1,000 Microsoft employees are running the next version of Windows Mobile, code-named "Crossbow,"
• 7,000 Systems Management Server v4 -- renamed to System Center Configuration Manager 2007-- installations are live.
• Over 3,000 machines running the next Microsoft Operations Manager release are also in production