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.
Cosmos Cloud: Cosmos "take the chore out of selfhosting, with automated maintenance and fully secured setup out of the box. It even integrates to your existing setup."

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Note: Any non-rolling release based distro is likely to have out-of-date versions.