Clutter is an open source (LGPLv2.1+) software library for creating fast, compelling, portable, and dynamic graphical user interfaces. It is a core part of MeeGo, and is supported by the open source community. Its development is sponsored by Intel.
Clutter uses OpenGL for rendering (and optionally OpenGL|ES for use on mobile and embedded platforms), but wraps an easy to use, efficient, flexible API around GL's complexity.
Clutter enforces no particular user interface style, but provides a rich, generic foundation for higher-level toolkits tailored to specific needs.