by Apple under Apple Public Src license. Many people don't know the OS base of Mac/iPad/iPhone/ipod is BSD Unix, compatible with Single Unix Spec. running on top of Mach 3 microkernel. Linux has HFS+ module which allows you to mount and read/write non-journaled HFS+ disk. However, the current version in most Linux distribution doesn't support HFS+ greater than 2T partition, and readonly on journaled filesystem. Perhaps better support will come out later. Mac also start support ZFS, so hard to say what the future is gonna be.