Yup, XMonad. To be honest, the original reason was that it was recommended to me by a friend. I had a look at it, tried dwm and awesome as well, talked about this with some other users, and settled with it. I worked on my xmonad.hs, and am now really, really happy with it. Also I'm fascinated by functional programming languages, and plan to learn Haskell.
Whenever I'm forced to use a stacking window manager, like on Windows, it seems so inefficient to me now.