Some of these are very funny (though many are not — we are dealing with amateurs, after all). The idea is to be filmed (often in a public place, like on the subway) “singing” along to music on your MP3 player, and then add the music to the video before posting it. Go to vimeo.com and search for clips tagged “lip dubbing” — works best on a high speed connection (with sound on). Joining helps — my DSL at home worked better when I joined and could get rid of the animated ads, so the bandwidth was devoted to the clips, but was much better where I first saw it, with a faster connection.
Here’s an example from Jakob Lodwick who I think started this:
Or — you don’t have to SING, just do SOMETHING. See http://vimeo.com/clip:138598
Thanks to mroth for telling me about this.