Chrome: It's difficult enough to keep track of all your open tabs, so when one starts playing an ad for mouthwash, it's a pain to root out the source. For Chrome users, MuteTab by Jared Sohn identifies the problem tabs, allowing for easy toggling of any unwanted noise.
The extension sits in your navigation bar, and when clicked, displays a pop-up menu with information on all your active tabs. From there, you can choose to mute all background tabs, or single out a specific source. The menu can also be accessed via right-click.
MuteTab isn't perfect, though. When used with YouTube, trying to mute all other tabs at once will sometimes mute your current tab as well. Kwame Opam at Gizmodo also points out that the search for specific plug-ins can get complicated.
Still, when you're browsing sites that autoplay their ads, MuteTab is pretty useful.