QuickTime and iTunes. I used to watch tv or a movie on one of my displays while coding on another. Instead I got to watch linen one screen while watching a movie (double whammy here because even showing pure black on the other screen would have been better since at least it wouldn't distract from the video).

I can't specifically recall, but I think the same was true of full screening video content from Safari (perhaps technically a "plugin feature", but maybe html5 video was around?), but QuickTime was 100% an apple regression.