i3wm.org
i3 is a tiling window manager written from scratch for GNU/Linux and BSD operating systems. Its source code is free and open source under the BSD license, and it is primarily targeted at advanced users and developers. The project uses libxcb instead of Xlib, supports Xinerama multi-monitor setups, and arranges windows with a tree structure. It also provides an IPC interface for other programs and documents its internal workings.