I’ve known that ZFS was a revolutionary filesystem, but never understood the details. Now there is an article that explains why ZFS is so desirable, and does so well.
Apple started supporting ZFS read-only in Leopard, and has released beta versions of Leopard with writable ZFS.
FreeBSD committed ZFS to the 7.0 tree in April of 2007. There is an excellent article that describes how to install FreeBSD 7.0 with ZFS. The FreeBSD Project also has a wiki page that describes the current state of ZFS under FreeBSD, and has some nice links about ZFS.
So why isn’t ZFS in the Linux kernel tree? Because the license for ZFS, the Sun CDDL, conflicts with the Linux kernel’s GPL license. There was an interesting discussion on the Linux Kernel Mailing List (lkml) summarized at kerneltrap.
One way to avoid the license issues is to run Linux inside a Solaris zone; while the Linux system is not aware of the filesystem used as the backing store for the zone, the Solaris system could use ZFS as the zone’s filesystem.
UNIX Administratosphere 
You can use zfs through fuse in Linux. The performance isn’t as good, as expected, but the administration is easy. I really love to see a port for HPUX. Sigh.