50 Mac Essentials #4: Perian

Perian calls itself "the Swiss Army knife for QuickTime," a description that's pretty much spot-on.
Technically, Perian is a "QuickTime component" and it's a preference pane rather than an application (which means that after installing, you'll find it in System Preferences, not in your Applications folder).

But if the technical details leave you blurry-eyed, all you need [...:]