Perian: Make QuickTime play almost any video format
If QuickTimeis your default media player, then Perian is the tool you need to make sure it plays any media file type you want. It will not work with QuickTime 10 or above and since it's no longer under development, never will.
This discreet little component adds support to a wide range of formats including MKV, FLV, DivX, and WMA or even less known formats like Xiph Vorbis or Flash ADCPM. It also offers subtitle support for SSA and SRT so you can enjoy films in their original version in Quicktime. Users also like that Perian offers AVI support for H.264 and audio files like MP3 or AAC.
Perian is very easy to install and works automatically once it's set up. While VLC Media Player is usually the recommended media player on Mac because of its wide support for all sorts of media file types, having Quicktime with Perian is also an excellent choice to play any sorts of media.
An excellent and free QuickTime component, Perian lets you enjoy almost any media format, including subtitles and a number of audio files. Highly recommended.