[rfreiberger]

I'm not sure if this is the same as the DJ apps that remove vocals and other instruments but there seems to be a surge of apps that have this feature. It's impressive how well some of these work and I believe they are mostly based on an open source project (I can't recall the name).

[twosdai]

There's a neat trick you can do with songs that don't have much reverb. If you load a track. Clone the channel into a second one and then just "flip" the wave form it will remove all the vocals.

Not perfect but it's pretty damn good.