When you combine the wonders of Melody Assistant/Virtual Singer (free-to-try shareware which is quite inexpensive, powerful, and freakily designed) with all the freely available midi-karaoke files out there then . . . well, you get endless potential for magical moments. At least, you do if you're me. Here are a few little performances I've staged with minimal effort: Exhibit A, Exhibit B, Exhibit C.
Gregg, that last one's for you.
[UPDATE: Synth-like voices ID'd in nature. See here and here.]