ZFS is the name of the filesystem created by Sun Microsystems for the Solaris operating system. It has also been ported to other operating systems such as Mac OS X, FreeBSD, and Linux. Thanks to problems with compatibility between ZFS's CDDL and Linux's GPL licensing ZFS has been restricted to a user-space implementation and a third-party native kernel module with no hope of being included in the mainline kernel.
At one point I tried to move my home system from being Linux on top of Xen using CentOS to Linux on top of xVM Server (Sun's version of Xen) using Solaris Express Community Edition. I did this primarily to take advantage of the features offered by ZFS.
After using it for a while, I discovered some of its limitations. These are mainly regressions in features that I noticed while going from LVM to ZFS.