This will probably be most appreciated by Drischord, but in case he hadn't seen it, I figure we can be the umpteenth music-geek blog to post about this.
I saw it at the AV Club but the videos are at the Wired Magazine site.
Basically this guy (who is an actual guitarist) took videos of "guitar gods" wanking away on their axes, and overdubbed his own playing, which is basically a more horrible (though in some cases only slightly less so (See Malmsteen, Y.)) version of that wanking. At first I didn't get it, but by the time I got to the Santana one, I was laughing so hard I was crying. I'm not sure if it was the eerily impressive way he tracks their hand movements with his own playing, or the fact that the accompanying instruments are only filled in when they're actually on screen (especially Ozzy Osbourne's clapping), but something about it is hilarious. I think it's because of the faces and exaggerated body motions the guitarists make. Having that accompanied by awful playing just makes me laugh.
Definitely worth checking out, especially the aforementioned Santana, the Yngwie Malmsteen (and orchestra) one, the Van Halen one, and the Slash one.
The postscript is that these were on youtube but of course someone had them taken down because of copyright violations (it's their policy to just take something down when given notice, which places the burden on the original contributor to show that it's not a violation, which probably makes sense from youtube's perspective, but it's still shitty).