The NetApp NAS box software stack, including WAFL, only ships on their own FreeBSD variant. I have no idea whether there internal developers can also use it on other host OSes. On the other hand, that software is not free, nor does it ship as software; you have to buy the NetApp box to get the software.
There are probably several other FreeBSD based appliances, meaning their software only uses FreeBSD.
I think what you meant to ask is: Is there any freely available user application software that only runs on FreeBSD? Not to my knowledge.