wmii

Anselm R. Garbe released wmii — "window manager improved 2" — in 2005. Windows are arranged in columns, as in Plan 9's acme editor, and each column can show its windows stacked, maximised or tabbed. The unusual part was the interface: wmii exposed its state as a virtual filesystem over the 9P protocol, so the status bar, key bindings and rules could be written in any language capable of reading and writing files, which in practice meant a shell script.

Garbe eventually decided wmii had grown too big and wrote dwm in 2006 as a stripped-down successor. wmii is still around, but has mostly served since then as a source of ideas for others, i3 among them.