Regularly:
Windows 7: Main System and netbook.
Windows XP: Work, and a virtual machine on the Main System.
Android 2.2: HTC smartphone
Honourable mentions:
Windows 1.01, MS-DOS 3.33, Windows 3.11, MS-DOS 6.22, Windows 95, Windows 98, Windows 2000, Windows Server 2003, Windows Vista, Windows Server 2008R2, Windows 8 (Pre-release versions), GEM 2.0, System 7, Mac OS 8, iOS (borrowing original iPhone), Fedora 11,