VST Plugins are high quality sound processing modules used in many commercial audio editors. The difficult part has been finding free apps which can load VST plugins. Audacity has a module, but it isn't ready for prime time.

Christian Budde has developed a VST interface for Winamp which works nicely.

You can get it here:

http://www.kvraudio.com/get/1946.html

Here's a few dozen free VST plugins to get you started. Start with the 1 Knob Compressor below, as it is, well, 1 Knob

http://www.betabugsaudio.com/dl.php?id=1 1 Knob Compressor

Another set which is highly recommended are the DigitalFishphones Fillet Pack http://www.digitalfishphones.com/main.php?item=2&subItem=5

The other 80+ free VST audio enhancer plugins:

http://www.kvraudio.com/get.php?mode=results&st=adv&soft=e&type%5B%5D=19&type%5B%5D=23...&sort=2&rpp=45

* Using the disk writing plugin that comes with Winamp, you can easily save your enhanced file to .WAV then convert to .AC3/MP3/MP2 with your app of choice.
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Here's an MP3 audio example. I'm punching the plugins in and out, you'll hear the first group at :20 for ten seconds, then I put in various groups of filters at 5 second intervals. Most of them are various limiters and spectral exciters, plus bass enhancers and stereo wideners.

vst-example.mp3