Have you written an nbdkit plugin that you're willing to share?
Email <guestfs@lists.libguestfs.org> to get it added to this list.

* XVA (XenServer Virtual Appliance)

This simple plugin written in Perl lets you read these files and
reexport them as NBD.  It does not require that the XVA is unpacked:
https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg02971.html
https://lists.gnu.org/archive/html/qemu-devel/2017-11/plHKJ062x5Uo.pl

The whole thread is worth reading because several different approaches
to dealing with XVA files are presented:
https://lists.gnu.org/archive/html/qemu-devel/2017-11/threads.html#02839

* Minecraft "block block device"

https://github.com/williambl/blockblockdevice

"Block Block Device is a Minecraft mod and NBDKit plugin which work
together to allow you to create block devices on your computer which
are backed by redstone in a Minecraft world."

* Tom7's Hard(er) Drives

http://tom7.org/harder/

This is a C++ nbdkit plugin which stores data "in flight" in ping
packets.

* Proxmox backup server plugin

https://github.com/abbbi/cpbsnbd

Export your virtual machine disk backups from a Proxmox Backup Server via NBD.
