With the Ion 3 stable release around the corner, alpha geeks could be tempted to look for another alternative. Well, there is xmonad written in Haskell, and its development seems quite active, with lots of extensions available, even though it's only at its 0.3 release!
For those interested, xmonad's name comes from the concept of monads in functional programming languages, of which you can find a definition on wikipedia.
Although I'm an Ion 3 addict, I'll keep an eye on it. Along with StumpWM, written in Common Lisp, even if its development doesn't seem as dynamic as xmonad's.