This is a quick and dirty list of some of the more important features of gephex and of the most important features of the development branch gephex-0.5.

gephex-0.4 (stable branch)

The current stable release here is 0.4.4.

free (GPL) and cross-platform (GNU/Linux, Win32, experimental Mac OS X support )
graphical user interface (written in QT)
easily extensible with plugins
client server architecture
support for video files (ffmpeg, libavifile, video for windows, directshow)
ports of most EffecTV effects
vloopback support (input and output)
webcam and tv card support (v4l and directshow)
midi/joystick input support
audio support (capturing from soundcard, simple beat detection, simple mixing, audio synthesis, audio output)
frei0r plugin support

gephex-0.5 (development branch, not released yet)

For an overview of the design of gephex-0.5 have a look at Gephex05Discussion.

scriptable in scheme
freeframe plugin support
improved stability
revised, simpler and more powerful architecture
user interface better suited for live performance