For the past couple years, this guy wzauscher has been posting videos of himself singing various classical works in multiple parts. For example, the first one he did appears to have been "Bach Bach Bach," in which he sings a five-part Bach piece with himself as all five singers and the conductor. Fun and impressive.
He's done several others since then, and other people have picked up the idea. (I'm not sure whether it was wzauscher's idea originally or whether he got it from someone else.) One of my favorites of those I've seen so far is Paulinho Winterle singing "Canon em D," better known in English as Pachelbel's "Canon in D," taking six parts in six separate little sub-windows, and being generally silly.
Meep meep meep!
(And that led me to a classic Muppet Show bit: Beaker and the Swedish Chef and Animal singing "Danny Boy." But I'd better stop there before I end up watching every Muppet video ever posted. But for more of Beaker, you can always search for videos tagged beaker.)