Xwayland
An X server that is itself a Wayland client, so old X programs can run on a Wayland desktop without noticing the switch.
Wayland was meant to replace X11, but nobody was going to rewrite thirty years of software. The answer was to have the compositor launch an X server on demand, built from the same codebase as the X.Org Server, which instead of drawing to the screen hands each X window to the compositor as a surface. The X program sees an ordinary display; the compositor sees one more client.
Xwayland was merged into the X.Org server in 2014 and has shipped as its own package since 2021 — in practice the only part of X.Org still under active development. The seams show wherever X allowed more than Wayland does: screen grabbers, global keyboard grabs and tools like xdotool only see other X windows, and scaling on HiDPI displays can turn blurry.