i3 is a dynamic tiling window manager. Its key features are correct implementation of Xinerama (workspaces are assigned to virtual screens, and it does the right thing when attaching new monitors) and XrandR support (which is still unfinished). Both horizontal and vertical columns can be used in tiling. There is a special focus is on writing clean, readable, and well documented code. i3 uses xcb for asynchronous communication with X11, and has several measures to be very fast. i3 is primarily targeted at advanced users and developers.
Bluetile is a tiling window manager designed to integrate with the GNOME desktop environment. It provides both a traditional, stacking layout mode as well as tiling layouts where windows are arranged to use the entire screen without overlapping. Bluetile tries to make the tiling paradigm easily accessible to users coming from traditional window managers by drawing on known conventions and providing both mouse and keyboard access for all features.
x9wm is a clone or fork of the 9wm and w9wm X window managers. It is a light alternative for the Mac OS X desktop. All of its source code is contained in a single file. It supports an alterate red colored cursor. It is very light on resources, quite fast, very simple, and easy for long programming, editing, or Web work sessions. You can blend it with Nitrogen and Wbar to create a simple but elegant interface without iconic or stylistic clutter. It does not decorate windows with borders, and it is modal, controlled with the mouse.
xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard; a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings, and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens.