For those out of the GNOME loop, Mutter is an OpenGL-using window and compositing manager that leverages the Clutter tool-kit. The Mutter window manager is to play a critical role in GNOME 3.0 that is due out in September. In current versions of GNOME, Mutter can be used as an alternative to Metacity for which Mutter is derived. Other key packages being worked on for GNOME 3.0 include the GNOME Activity Journal, Zeitgeist, and the GNOME Shell.
The Mutter 2.29.0 release announcement can be read on the gnome-announce-list.