xmonad (all lower case) is a tiling window manager built in the Haskell programming language (which is interesting enough to qualify for inclusion in this list). So what is a “tiling window manager”? Simple - no overlapping windows. That may take a moment to sink in for those used to KDE, GNOME or modern Windows/Mac. You won't be grabbing any title bars to drag windows around, because window title bars don't really exist here. But virtual desktops and an intensely high level of customization mean you'll be quite productive. And did I mention how much faster your system will feel?