Mac OS X Leopard incorporates now a previous versions feature called Time Machine – similar with something that we added in Windows Server 2003 a while back, and recently in Windows Vista as well.
If you just read the text at http://www.apple.com/macosx/leopard/timemachine.html it looks a lot with what Vista already offers as well – automatic backup of your whole system at night, ability to use an external drive for your backups (Vista can also backup to a DVD-RW), ability to restore any file by going back in time, etc.
However, their UI is very interesting – it is a Flip3D-like browser of previous versions of your file system.
P.S. Time Machine? Funny enough – when we designed the Previous Versions feature in Windows Server 2003 back in 2001, the code name of our product was Timewarp.