Mac OS X
Mac OS X is the latest version of the MacOS operating system. It has been significantly updated from previous versions, having a Unix-like operating system at its core, Darwin, based on BSD, and as such, can run a lot of software targeted for Unix or Unix-like systems. Mac OS X's kernel is XNU (which is based on Mach and FreeBSD kernels).
Mac OS X has its history in the OPENSTEP/NeXTSTEP operating systems, as its main software library Cocoa is based on the same software library found in OPENSTEP.