Projects incorporating mergerfs directly in some way

Lakka.tv: A turnkey software emulation Linux distribution. Used to pool user and local storage. Also includes my other project Opera. A 3DO emulator.
OpenMediaVault: A network attached storage (NAS) solution based on Debian Linux. They provide plugins to manage mergerfs.
CasaOS: "A simple, easy to use, elegant open source home cloud system." Has added initial integration with mergerfs to create pools from existing filesystems.
ZimaOS: A more commercially focused NAS OS by the authors of CasaOS at Ice Whale.

Software and services commonly used with mergerfs

snapraid
rclone
- rclone's union feature is based on mergerfs policies
ZFS: Common to use ZFS w/ mergerfs
UnRAID: While UnRAID has its own union filesystem it isn't uncommon to see UnRAID users leverage mergerfs given the differences in the technologies.
For a time there were a number of Chia miners recommending mergerfs
cloudboxes.io

mergerfs can be found in the repositories of many Linux (and maybe FreeBSD) distributions.

Note: Any non-rolling release based distro is likely to have out-of-date versions.