Ok, here's a really simple one (of terrible video quality, though not as bad as the splash image implies). This was obviously put up after Kingdom Hearts: Final Mix was released but before Kingdom Hearts 2 was technically announced. If you were a fan of the game, you know how it was, you got all the Ansem Reports, it triggered a post-game cutscene giving you the first teasers about the next major installment. This video is that cutscene, but with one appreciable alteration: Some guy decided to combine it with Final Fantasy VIII's song "Liberi Fatali" (you know, the song that played in FF8's opening cutscene).
And the thing that made it stick in my memory (besides the fact that I just plain like the song) is that for doing so little, the music and video sync up surprisingly well. If you were to tell me that the animators had designed it to be played with that music, I probably could believe it.