Awesome Window Manager
The Awesome Window Manager, awesome-wm, is a tiling window manager for Linux PCs that can be used as a standalone X-session or as part of a Desktop Environment, e.g. Gnome, KDE, etc. It's a great lightweight alternative to a full-fledged Desktop Environment, and it's one of the few GUI sessions available that handles dynamic desktop-layouts on multiple monitors/displays quite well. The image below, for example, shows a PC with a built-in display and six separate desktop layouts, along with an external display with its own set of six desktop layouts.