And if an artist releases instrumental or a capella versions of their track, you can usually use this to get the other. Only works if they're exactly the same as the original of course.

Another trick is to invert one of the stereo channels and add it to the other one: this eliminates everything that was mixed in with neutral pan. That's typically where the voice is. It will also remove e.g the drum kick, but this can be salvaged with a low-pass filter. Note that you end up with a mono track.

While on the subject of these tricks -- disconnect the ground from the audio out, and tie the speaker grounds together. This causes the speakers to use each other as ground, which in effect causes the middle voice to cancel out.

I used that trick to get a cheap rear channel from my thrift store stereo back in my high school days.

That's really interesting. I never knew how they did that. Canceling out the frequencies of the voice makes complete sense.