VistaSwitcher is an elegant and powerful task management utility for Windows OS. Once installed, it replaces the default Windows Alt-Tab dialog with a nice box that shows a list of all running tasks, with their names and preview snapshots, and enables the user to take actions on...Discontinued
The program is no longer updated. Last version, 1.1.5, released in May 2012, can be still downloaded from the official website.
Skippy-XD is a full-screen task-switcher for X11. You know that thing Mac OS X, Compiz and KWin do where you press a hotkey and suddenly you see miniature versions of all your windows at once? Skippy-XD does just that.
KWin is a window manager for the X Window System, and is in the process of becoming a Wayland compositor. It is an integral part, and the default window manager of the Plasma Workspaces, but it can also be used on its own or with other desktop environments.
Sawfish is an extensible window manager using a Lisp-based scripting language. Its policy is very minimal compared to most window managers. Its aim is simply to manage windows in the most flexible and attractive manner possible.