From Jonathan Coulton's blog (you know, the Codemonkey guy): Kutiman, an Israeli musician, has put together an astonishing funk piece by sampling pieces of YouTube videos. You can see the first video below, for more visit thru-you.com (also, the site is a visual experience in itself, definitely worth visiting).
An other interesting fan-made music video is the one below. Regardless of whether you like the style or not, it is incredible what people can put together even on a low budget.
Finally, via The Old New Thing blog comes this Michael Jackson tribute video:
PS. Speaking of Friday the 13th, I lost this message because I forgot to turn off Fiddler, and the, when Live Writer complained about SSL errors, I hastily closed it, without saving the draft, so this is the second go :-)