Originally written by Apple as part of OS X 10.6 Snow Leopard, Grand Central Dispatch is, in very simple terms, an abstraction of operating system threads that allows a programmer to make use of parallel programming with fewer lines of code, which should in theory create more efficient code with fewer opportunities for bugs.

The FreeBSD developers have now merged Grand Central Dispatch support in to the FreeBSD 8-STABLE branch, and it should be supported in the main branch with FreeBSD 8.1 to be released in the coming months. You can find the original mailing list announcement here: http://lists.freebsd.org/pipermail/freebsd-stable/2009-November/052738.html

For more information on Grand Central Dispatch, have a look at the following resources.

http://developer.apple.com/mac/articles/cocoa/introblocksgcd.html

http://libdispatch.macosforge.org/

http://en.wikipedia.org/wiki/Grand_Central_Dispatch