From Wikipedia:
Quote Originally Posted by Wikipedia
Thanks to its permissive licensing terms, much of FreeBSD’s code base has become an integral part of other operating systems such as Apple's OS X that have subsequently been certified as UNIX-compliant and have formally received UNIX branding.
and
Quote Originally Posted by Wikipedia
Other operating systems such as Linux and the RTOS VxWorks contain code that originated in FreeBSD. Debian, known primarily for using the Linux kernel, also maintains GNU/kFreeBSD, combining the GNU userspace and C library with the FreeBSD kernel.[37] Darwin, the core of Apple OS X, borrows FreeBSD’s virtual file system, network stack, and components of its userspace. The OpenDarwin project (now defunct), a spin-off of Apple’s Darwin operating system, also included substantial FreeBSD code. Thanks to the permissive FreeBSD License, much of FreeBSD now also forms the basis of Apple OS X and OS X Server.
It's not the GUI part that's from BSD of course ...