Is Mac OS X open source?

Darwin is open source operation system from Apple and it's core of Mac OS X. But Darwin hasn't Mac OS X GUI.

• Darwin is like Linux, FreeBSD, etc...
• Mac OS X GUI is like KDE, GNome, etc...

Others have already explained that Darwin is open source. It's Mac OS X's kernel, similar to how Linux is the kernel of a Linux distro. Mac OS X's windowing system (called Quartz Compositor) is not open source.

You also ask about other open source parts of Mac OS X:

Besides the kernel, What about the various other pieces? The X server? Window Manager? File explorer? etc. What's open source and what's not?

One of the most prominent Apple open source projects is probably WebKit, which was originally a fork of KHTML and KJS. WebKit is used by a huge number of browsers.

Another Apple open source project you may have used even if you don't use Mac OS X is [Bonjour](http://en.wikipedia.org/wiki/Bonjour_(software)), Apple's implementation of Zeroconf.

Apple also owns CUPS, a printing system that is used by a number of operating systems.

Apple also has a number of other open source projects, such as a streaming server, and they contribute to many more open source projects, such as gcc. You'll find more at opensource.apple.com and Mac OS forge. In addition to that, Mac OS X ships with hundreds of open source components from Apache to ZFS, most of which are not owned or maintained by Apple.

The kernel is open source; the user interface and all the applications that the OS comes with are not.

Apple does maintain some open source projects, but for the most part, everything besides the core kernel is closed source. For example, WebKit (the HTML rendering engine behind Safari, Chrome, and other browsers) is open source, but the Safari browser itself is not. Finder, Spotlight, and most (if not all) of the applications that come with the OS are closed source.