mergerfs

Projects using mergerfs

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."

snapraid: a backup program designed for disk arrays, storing parity information for data recovery in the event of up to six disk failures.
rclone: a command-line program to manage files on cloud storage. It is a feature-rich alternative to cloud vendors' web storage interfaces. rclone's union feature is based on mergerfs policies.
ZFS: Common to use ZFS w/ mergerfs. ZFS for important data and mergerfs pool for replacable media.
UnRAID: While UnRAID has its own union filesystem it isn't uncommon to see UnRAID users leverage mergerfs given the differences in the technologies. There is a plugin available by Rysz to ease installation and setup.
TrueNAS: Some users are requesting mergerfs be made part of TrueNAS.
For a time there were a number of Chia miners recommending mergerfs.
cloudboxes.io: VPS provider. Includes details on their wiki: on how to setup mergerfs.
QNAP: Someone has create builds of mergerfs for different QNAP devices.

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

Note: Any non-rolling release based distro is likely to have out-of-date versions.