RollTube – the rick roll firefox extension from fffffat labs on Vimeo. labs writes:

RollTube is a Rick Roll Firefox Extension that switches every YouTube video with the infamous Rick Roll video. The experience has been described as “like the whole internet ganging up and rick rolling you at the same time”. We say: great to install across all the machines in your office / lab / classroom. RollTube is cross platform and needs no special admin rights to install!

RollTube Firefox extension (open source, too) – Link.