Notion is a tiling tabbed window manager for X. This means the screen can be freely divided into tiles that contain the client windows. Multiple client windows can occupy the same tile; in that case they will be tabbed, much like browser tabs. This way, windows will not overlap and it becomes easier to manage your screen real estate. Its functionality can be configured and even extended with Lua scripts. Various scripts are available, such as for NetWM features and multihead support.
WMFS (Window Manager From Scratch) is a lightweight and highly configurable tiling window manager for X. It can be configured with a configuration file (made with LibConfuse), supports Xft (Freetype) fonts and is compliant with the Extended Window Manager Hints (EWMH) specifications.