This is a present from Mac Amour to all its visitors, it’s Perian 1.0b2, you can download it from HERE.
Perian enables QuickTime® application support for additional Media Types including:
* AVI and FLV
* 3ivX, DivX, Flash Screen Video, MS-MPEG4, Sorenson H.263, Truemotion VP6, and Xvid
* AVI support for: AAC, AC3 Audio using A52Codec, H.264, MPEG4, and VBR MP3

Perian 1.0b2 supports .mkv and is a bit faster to me.You can get the version 0.5 from Perian website.

You can see all supported formats here.
And I also advise you to take a look at the Google Discussion.

Apple has added Xvid to its online QuickTime Components list. The Xvid-for-QuickTime component, allows you to play and encode Xvid video. Apple states:
The Xvid component for QuickTime allows users to play & encode Xvids in QuickTime Player. Xvid is a video codec library following the MPEG-4 standard.
In theory, this means that the Apple TV could also support Xvid video playback.

Go to my My iDisk and you’ll find the Plug-In to play your Ogg’s on iTunes!It’s Great!