Wanting to provide networked Time Machine backups for a number of macs I decide to try the Iomega StorCenter ix2-200d. This has a wide feature set and a low entry price. Of course, you can build your self a FreeNAS based system that will offer much of the same functionality but the automatic support for Time Machine is what did it for me.

The Apple open source site at http://www.opensource.apple.com has a fantastic browsable repository of open source OS X components. Notables are (check for more recent versions on the source site):

libc - C libraries including stdlib

xnu - the base OS X kernel

objc4 - the objective-C runtime