ZFS is gaining interested in FreeBSD land so I guess the source for it must be available...
Not just FreeBSD.. It's somewhat popular on Linux too (
http://zfsonlinux.org/). On both it's pretty darn stable. Due to its license, though, it cannot be put into the Linux kernel.
I think Linux and FreeBSD have both gone down different paths and have slightly different ZFS features (although I think the pools + filesystems can be set to a level that maintains compatibility). Sources for both are available though.