Thursday, September 1, 2011 at 7:52PM iTunes Visualizer plug-in for Mac OS X. Made this a while back. It's really hard to screencapture this visualizer, but there is a lame attempt:
Better to get the real plugin and run (if you have a Mac!)
Download: SoundThing.bundle.zip
Another iTunes visualizer is Klangfarbe, it has a better screencap video and more install instructions.
Saturday, May 8, 2010 at 1:34PM 
This plug-in is still in beta and does not work with all graphics cards, please let me know if it works with yours!
In my first visualizer - AlphaOmega - I tried to envision a form or geometry for the music. This one, Klangfarbe deals more directly with visualizing the current audio as colors, hence the name which is German for "timbre" but also means "color of the timbre" or "color of the sound".
License
Free for personal, home, non-commercial use. For other use, please contact peder /dot/ norrby (at) gmail /dot/ com. This free software is provided as is, with no guarantees whatsoever.
Download
TrapcodeKlangfarbe_BETA5.zip
It has a few settings:
Press "m" key to toggle modes, there are three modes and the first two are animated (they change slowly over time)
Press "f" key to change particle feather / out-of-focus effect.
Install help
1. Unzip the .zip file (double click it) if needed.
2. Place TrapcodeKlangfarbe in the iTunes Plug-ins folder. Normally this is:
(username)/Library/iTunes/iTunes Plug-ins
.
3. Restart iTunes
.
4. In the menu select View>Visualizer>Klangfarbe.
5. Play a nice tune.
6. Hit Command-T to start the visuals!
Friday, December 11, 2009 at 3:20PM I'm working on a new iTunes visualizer plug-in. The working name is "Milky Way". Here is a video that shows how it currently looks:
Music: Anna Ternheim "Shoreline"
Please download and test the plug-in (Mac only for now) MilkyWay_BETA4.zip and post some feedback as comment to this post!
Install help:
1. Unzip the .zip file (double click it) if needed.
2. Place MilkyWay.bundle in the iTunes Plug-ins folder. Normally this is:
(username)/Library/iTunes/iTunes Plug-ins
3. Restart iTunes
4. In the menu select View>Visualizer>Milky Way
5. Play a nice tune
6. Hit Command-T to start the visuals! (And Command-H to enter fullscreen)
Here is an earlier iteration of the plug-in:
Music: Autechre "Kalpol Introl"
You can find the older iTunes plug-ins from Trapcode here:
Trapcode SoundThing
Trapcode Klangfarbe
Trapcode AlphaOmega