Its actually YouTube that made me start using AdBlock Plus. For a period, before they put ads in the video themselves, they ran them (only) on the side. And for a period they had ads from Sony that made sound. Do you have any idea how annoying it is to be watching a video only to have a random, annoying song start blaring?
These days I leave it on by default and disable it on sites that I support. But still not YouTube, I still haven't forgiven them for their past...screw ups =__=