This is pretty old, I think, but I haven't seen it mentioned on here.
Anyway The Legion of Rock Stars is basically this:
A bunch of people put on noise-cancelling headphones and listen to the same song. They play along with it and record themselves. Then they play it back along with the original song's music video.
A bunch of these are on Youtube, but most of them aren't really that funny. The best ones had me laughing pretty hard, though.
So far these are my favorites:
Danzig - "Mother"
and
Journey - "Anyway You Want It"
These seem to get taken down sometimes so if the links don't work, just search for "legion of rock stars" at Youtube. Soem of the other ones are OK, but I can go ahead and tell you that "Mmmbop", "Rocket Man", and "Let's Spend the Night Together" are a waste of time.