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Audacity
Audacity is an audio editing program. It's freeware, but may require codecs to work with .mp3 files.
Usage
Its functions include: trimming sound files with unlimited detail, reversing, changing pitch and tempo, inverting sound phase, adding various effects such as reverb, disortion and wahwah, and much more. It can load multiple audio files and play them all at once. It's often used along with Windows Movie Maker in order to combine multiple audio tracks (e.g. music + effects), something that WMM itself cannot do.
A nice trick
Import a stereo audio file into Audacity. Right click on the panel at the left of one of the tracks (there should be 2, one for each ear) and select "Split Stereo". Then, right click on the same panel on each track, and in both cases select "Mono" instead of "Stereo". Select (ctrl+a) ONE of the tracks and choose "Invert Phase" from the effects menu. If the original audio file had good quality, chances are that the vocals are removed from the song and you've got a karaoke version. This worked for me only with a few songs, most of them were ripped straight from a CD.
