Libvisual is an abstraction library that comes between applications and audio
visualisation plugins.

Often when it comes to audio visualisation plugins or programs that create
visuals, they depend on a player or something else; basically, there is no
general framework that enabled application developers to easily access cool
audio visualisation plugins. Libvisual wants to change this by providing an
interface towards plugins and applications; through this easy to use interface
applications can easily access plugins and, since the drawing is done by the
application, it also enables the developer to draw the visual anywhere he 
wants.

The framework also allows you to morph to different plugins and mix two at
once; all kinds of neat tricks are possible using this method.

WWW: http://libvisual.sourceforge.net/