UnionFS

Unionfs is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems.

This strategy enables Docker's lightweight images, as only layer updates need to be propagated (compared to full VMs, for example).

[5] UbuntuLTSP, the Linux Terminal Server Project implementation for Ubuntu, uses Unionfs when PXE booting thin or thick clients.

unionfs-fuse is an independent project, implemented as a user space filesystem program, instead of a kernel module or patch.

[10] The Plan 9 from Bell Labs operating system uses union mounts extensively to build custom namespaces per user or processes.

mergerfs is a FUSE based union filesystem which offers multiple policies for accessing and writing files as well as other advanced features (xattrs, managing mixed RO and RW drives, link CoW, etc.).

[14] JailbreakMe 3.0, a tool for jailbreaking iOS devices released in July 2011, uses unionfs techniques to speed up the installation process of the operating system modification.