To quote the Clutter site:
Clutter currently features the following:
* Scene-graph of layered 2D interface elements manipulated in 3D space via position, grouping, transparency, scaling, clipping and rotation.
* Frame based animation engine providing path interpolation, transitions and other custom effects via Behaviours and Timelines.
* Scriptable JSON based layout and animation file support.
* Advanced input event handling including multiple pointing devices.
* Custom Pango renderer providing efficient internationalised UTF8 text rendering.
* Support for high end Open GL features such as Shaders and FBOs.
* Support for media playback with GStreamer, Cairo graphics rendering, GTK+ embedding, Box2D physics engine and WebKit web rendering are available via optional add-on libraries.
* Object oriented design via GObject with a familiar GTK+ like API.
* Runs on Linux, Windows and OSX with backend window system support for GLX, EGL, WGL, SDL and Cocoa.
* Support for mobile devices with fixed point internals and portability across Open GL, OpenGL ES 1.1 and OpenGL ES 2.0
* Developed in C, with language bindings for Perl, Python, C#, C++, Vala and Ruby.