KWin
KDE's window manager since 2000, which over the years became a compositor, an effects engine and a Wayland server.
KDE 1 had kwm. KDE 2 in 2000 brought KWin, written to follow the new window manager standards and with pluggable window decorations. Through KDE 3 it was a perfectly ordinary stacking window manager; with KDE 4.0 in 2008 it turned compositing, with wobbly windows, the cube and translucency as the demo effects of the day.
It later grew into a Wayland compositor, and Plasma 6 made the Wayland session the default in 2024. The same codebase still runs the X11 session. KWin is unusually configurable — per-window rules, custom effects and JavaScript scripts — which has let users build tiling layouts on top of a manager never designed for it.