I've been maintaining a YouTube channel dedicated to a cappella for a while now (username: chirojoel), but I firmly believe that three of the best videos on the 'net right now were posted about two weeks ago by user johnnyherb. They're the USC Reverse Osmosis set from the ICCA Finals in 2006. I encourage everybody to check 'em out:
http://www.youtube.com/watch?v=Dk7fOpkd494 -- Sugar, We're Going Down (Remember the backstage footage Joey brought us in the AU Vodcast? That was an hour before this on-stage performance)
http://www.youtube.com/watch?v=xdTsa9UKbcE -- Fix You
http://www.youtube.com/watch?v=lPLrqulJcRw -- I Would Do Anything For Love (But I Won't Do That)
For the sake of this being an actual discussion, though, what YouTube a cappella goodness have you guys discovered out there?