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.
euclid-wm is a minimalist tiling window manager for X11. It is designed to allow quick and easy management of numerous windows entirely from easy-to-learn, Vim-like key-bindings. It seeks to do two things in particular: balance the ease of use common among automatic-layout tiling window managers with the flexibility of manual layout window managers, and create a useful way to handle minimized applications.
DSWM (Deep Space Window Manager) is a tiling keyboard-driven X11 window manager. It is based on StumpWM code and is written entirely in Common Lisp and oriented for good usability with minimum startup configuration and good integration with Emacs. The project is under hard development, so it has many experimental features.