In the days of modern graphics hardware, fast computers, and enormous digital music libraries, most people have heard of or seen a "visualizer". However, most people using main stream media players such as Windows Media Player and iTunes are missing out on what a true visualizer is.
Edit (3/26/2012): Granted, this post was written back in 2010. In those days, iTunes was not nearly as cool as it is now. Since my primary computer is now a 2009 MBP, I've found myself using iTunes significantly more. Their new-style visualizer with the colored balls is pretty visually stunning. My point is, don't think I'm bad mouthing your favorite visualizer here. If, at the end, you still believe that iTunes visualizer is better than Milkdrop...by all means, keep using iTunes, you're probably on a Mac anyway =]
I can describe in words the difference, but I think a video conveys the message better than anything.
Here is your typical WMP Vis. Plugin:
And here are some of the 250+ different presets for milkdrop advanced visualization
When actually running, the detail is much higher
The Milkdrop visualizer is different because all of its presents use mathematical formulas based on real-time frequency information, the 'beat', and other variables directly from the current song. They're different every time and one can watch many times and never really get bored with them.
In my personal experience, milkdrop visualization has made listening to music all that much better (especially on XL monitors). I would recommend it to anyone that likes music in the least
How to get it
Download the lastest version of Winamp (free version)
Play some music and switch over the visualization tab
If you don't see a visualization tab, you may need to show the Media Library.
Space bar skips to the next preset!
Linux (UPDATE 03/26/2012)
Because of the lack of development on projectM, I now recommend that Linux users simply run Winamp within WINE. I have been running this sort of configuration on and off on my linux boxes and the quality is just brilliant. That said, projectM has just released what looks like a SLICK android app. If you have an android phone (I don't), check it out: https://play.google.com/store/apps/details?id=com.psperl.projectM
Make sure you have the latest hardware graphics drivers installed for your distro. (Make sure you can run GL stuff)
Download the winamp installer above and give it to WINE
There are some settings to tweak, but if you're lucky everything will just work out of the box. Otherwise...well hey, that's what you're running linux for anyway! Shoot me an e-mail if you're really stuck.