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 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.