XMonad is great. For anyone who finds it a bit daunting I recommend Bluetile[1]. It's a great introduction to the best window management system I've used (I really miss XMonad on OS X.)
Lion actually improved that a lot: Fullscreen apps integrated with Spaces gives a large part of what I use X<onad for. If they'd combined that with Windows 7's snapping of windows to half-screen, that would cover 75% of my XMonad usage. It's really a shame that XMonad is Linux-only. (Or that Firefox users expect Windows&OSX ports, depending on your perspective).