Stereo Tool is a Winamp plugin that can be used to manipulate sounds.
It contains a singleband compressor, an 8-band multiband compressor, a lowpass filter and a stereo image manipulator.

The compressors can be used to make the playback volume and sound of different recordings more equal, or to remove loud peaks from recordings.

The stereo image manipulator can be used to repair phase shift problems in stereo recordings and to increase or decrease phasing and channel separation separately. It can also be used to convert sounds from stereo to mono, while preserving the fullness of the sound.

The lowpass filter is useful if certain frequencies need to be filtered out, for example the FM stereo pilot tone at 19 kHz if you send the Winamp output to an FM transmitter.