Perian is a QuickTime component that allows native playback of loads of formats and codecs that it would otherwise struggle with, or not be able to play at all.

Normally you have to go hunting all over the place for components and plugins to get QuickTime to play files than other popular media programs such as VLC can handle without issues. Perian allows native playback so you can take advantage of all the other features of QuickTime and its heavy integration with the rest of OS X.

Its free an very simple to install, simply double click and it installs the PrefPane for you. Their website is simple but very good, and even has a couple of video tutorials of how to install and how to download videos from YouTube as .FLV and play in Quicktime.

Perian allows QuickTime support for the following media types:

• AVI, FLV, and MKV file formats

1. •MS-MPEG4 v1 & v2, DivX, 3ivX, H.264, FLV1, FSV1, VP6, H263I, VP3, HuffYUV, FFVHuff,
   

2. •MPEG1 & MPEG2 Video, Fraps, Windows Media Audio v1 & v2, Flash ADPCM, Xiph Vorbis (in Matroska), MPEG Layer II Audio
   

• AVI support for: AAC, AC3 Audio, H.264, MPEG4, and VBR MP3

• Subtitle support for SSA and SRT