FreeNAS can use ZFS as the filesystem. And this is what you want! Now, the actual configuration depends on the drives you have available.
For drives with the same, or very similar capacity -- ZRAID can be used. With 3 drives, ZRAID1, or with more, use ZRAID2 (the number is the number of drives which can be failed). ZRAID offers the capacity of the smallest drive, which may waste space. If all drives are (eventually) increased in size, more storage is added.
For drives with different capacity, ZFS offers the ability to keep a redundent number of copies of the data (eg. specify two copies, or three). Then, ZFS will duplicate the data onto multiple drives.
As well, ZFS continually monitors the drives and redistributes any failed areas, and ensures that no bit errors accumlate in the file system. ZRAID and multiple copies can be combined.
The main point of ZFS is to keep your data clean and safe from corruption.
As well, "fsck" is not needed -- it happens when you "scrub", which slows down the array, but doesn't leave it unusable.
If you have sufficient memory, ZFS can also "dedup" the blocks in your filesystem, merging identical copies of data (but copying/raiding to maintain data integrity). This feature takes a LOT of RAM (2GB per TB of disk, 32GB for 20TB of disk, and possibly more). Also, some ZFS versions offer encryption (not sure about the one in FreeNAS).
ZFS drives can be physically moved to another system, and used (eg. FreeNAS x86 to SPARC). Endian and format issues are correctly handled. Not a feature most people would ever use, but it is nice. ZFS is available on Solaris, BSD, Linux, Mac (well, used to be).
Also, ZFS support snapshots, which can be browsed.
Finally, ZFS has an eight year history in production.
In all, what's not to love?