Xmonad, one of the best tiling window managers out there. It is very easy to grasp for beginners, has a comprehensive documentation and is extremely powerful under the hood. The configuration is in Haskell, which makes it Turing-complete, but you can customize Xmonad even if you are not familiar with the language.
Xmonad is what finally made me decide to switch to Linux from OS X. I'd tried other window managers but none of them acted quite how I wanted them to. Xmonad with Xmobar and dmenu is an amazing combination.
Xmonad works so well that I even rejected to work with a second monitor. At least for me its currently faster switching to another desktop using capslock-<0-9> than physically adjusting my eyes to another display.