Some people have mentioned window managers, but nothing comes quite close in terms of extensibility and feature as dwm[0] and xmonad[1], which is a near dwm reimplementation in haskell with many more interesting features.
Both programs are configured by writing code, which compiles to a binary that you then use. Xmonad and xmonad-contrib are maybe more accurately described as haskell libraries that make writing your own window manager trivial. Highly recommend if you enjoy configuration, you can do some really cool stuff with it.