Clutter is not just a library, but also a set of projects that integrate with the GNOME platform.
Clutter-GTK
The Clutter-GTK library allows embedding a ClutterStage into a GTK+ application, using a specialized GTK+ widget called GtkClutterEmbed. Clutter-GTK also allows embedding GtkWidget into a ClutterActor.
Clutter-GStramer
The Clutter-GStreamer library provides classes to integrate a GStreamer pipeline into any Clutter application, for fast rendering and decoding of video streams coming from files or captured from a device.