Mutter: window and compositing manager for GNOME 3
GNOME 3 is the GNOME project's ambitious effort to take its desktop into the future. A key component of the desktop is the window manager, which defines much of the overall feel of the system. Thomas Thurman, the maintainer of Metacity—GNOME's current window manager—is looking ahead to "Mutter" as the window manager for GNOME 3. Metacity 2 will gradually be phased out in favor of Mutter; in GNOME 2.28 it will be an alternative window manager, while in GNOME 3, it will take over the reins from Metacity.
The GNOME Shell, responsible for the new user experience in GNOME 3, runs as a plugin for Mutter. Started as a fork of Metacity, Mutter uses the Clutter toolkit. Clutter does its rendering using OpenGL or OpenGL ES, so using it in Mutter makes hardware acceleration for the window manager possible. Meanwhile, Clutter has just announced its 1.0 release.