Most karaoke music are actually 2 files, the music .mp3 and the words .cdg. The best free way to play both of these is to use an open source media player that will play music and display words.
http://www.videolan.org
There is software that can suppress vocal tracks in regular mp3′s but sounds sort of muddy.
http://download.cnet.com/GoldWave/3000-2170_4-10001099.html?tag=mncol;1
Now if your really creative and want to try your hand at making karaoke…
http://download.cnet.com/AV-Video-Karaoke-Maker/3000-13631_4-10761156.html?tag=mncol;1
Or you can get just instrumental tracks. (quality varies greatly as usually not from original artist)
Hope this helps and have fun