- DVD Player: Preferences → Full Screen → Dim other displays while playing
- QuickTime Player 7 (old): Preferences → Full Screen → Background Color → Show on all displays
- VLC: Preferences → Video → Black screens in Fullscreen mode (should play everything that QuickTime Player can)
Update 2010-05-28: QuickTime Player 7 – which still comes with Snow Leopard – has the aforementioned preferences. Alas, the current version of it has not.