awesome
A tiling window manager that started as a dwm fork and became a Lua program with window management built in.
Julien Danjou forked dwm in 2007. dwm is configured by editing its C source and recompiling, on purpose; awesome went the other way. Since version 3 in 2008 it has been built on XCB rather than Xlib, one of the first major window managers to make the switch, and the entire configuration is a Lua script.
That makes the panel, widgets, key bindings and layouts code the user owns. The result is powerful and occasionally brittle: configurations have broken between major versions, and a syntax error can leave you staring at an empty desktop. The name is written in lower case, and the project would like it kept that way.