GStreamer is a multimedia framework for Linux with that can use plugins.
An Internet guide to the tools and software you'll need to receive and broadcast streaming media with Linux.
zgv, svgalib, and other Linux projects.
Ice Sound Manager is a Gtk-based sound events and sound theme manager for IceWM that does not require Gnome.
The Linux MP3-HOWTO.
"An Oscilloscope On Acid" - Cthugha also lets you save certain visualizations to keystrokes which makes it a great tool for VJs.
Cinelerra is a full-featured non-linear video editor for Linux.
Art of Illusion is a 3D-modeller, renderer and raytracer. It is written in Java and runs on Linux, Solaris, OS X, Windows and probably more.
An opensource multimedia library to play, edit and author Apple Quicktime movie files.
GPL musical instrument tuner, originally conceived to tune electric guitars.