Mac: Perian

Perian didn't just work in QuickTime Player. Because it installed at the system level (into the QuickTime library), it enabled these formats in any application that utilized QuickTime:

Released during the era of Mac OS X Tiger and Leopard, Perian solved one of the biggest headaches for Mac users: the inability to play popular video formats like AVI, MKV, or FLV using the built-in player. perian mac

Since Perian is now legacy software (discontinued years ago), a detailed guide must cover its history, how it worked, why it was necessary, and—most importantly—what you should use today instead. Perian didn't just work in QuickTime Player