OS/2 was another one using multiple rings. Interestingly VirtualBox takes advantage of x86's multiple rings: http://www.virtualbox.org/manual/ch10.html#idp1262

OS/2 was another one using multiple rings. Interestingly VirtualBox takes advantage of x86's multiple rings: http://www.virtualbox.org/manual/ch10.html#idp12629776 http://www.virtualbox.org/manual/ch10.html#idp12629776 (which contains other interessting details about VirtualBox, like the disassembles and patches code running in the VM in certain cases)