FreeNAS (+FreeNAS) is excellent but it lacks the HAST (Highly Available STorage) feature, However, TrueNAS has this feature integrated. FreeNAS is a FreeBSD-based (nanobsd) open source Network Attached Storage Platform developed by iXsystems, that supports file sharing across Windows, Apple, and UNIX-like systems.
Since FreeBSD has already integrated the HAST in base system, this feature can be added to FreeNAS manually.
Follow these steps to setup a HAST in FreeNAS: Configure Highly Available STorage on FreeNAS.

The latest builds of NAS4Free actually include HAST+CARP support directly in the administration web interface. NAS4Free is a fork of the mature FreeNAS 7 codebase before the rewrite by iXsystems.
Thanks for the comment, Jeff. I wasn’t aware of that.