common-lisp
From the Stump Window Manager homepage:
Stumpwm is a tiling, keyboard driven X11 Window Manager written
entirely in Common Lisp.
If you're tired of flipping through themes like channel-surfing, and
going from one perfect-except-for-just-one-thing window manager to
another even-more-broken-in-some-other-way then perhaps Stumpwm can
help.
Stumpwm attempts to be customizable yet visually minimal. There are no
window decorations, no icons, and no buttons. It does have various
hooks to attach your personal customizations, and variables to tweak.
* Hack the good hack
* Debug your good hack
* Customize your window manager
While it's running. That's right. With a 100% Common Lisp window
manager there's no stopping the hacks. Just re-eval and GO!