Have you ever wished QuickTime would just play all those random formats you've got? That you didn't have to resort to other programs, or worse converting everything, just to get them to play? You need Perian.
We've covered Perian a lot in the past, and for good reason -- it's a one stop-shop media plug-in that just works. Installed as a Preference Pane, it essentially gives QuickTime support for a myriad of file formats, video and audio codecs, and subtitles. Included in the mix is playback of MKV files with H.264, DivX and VP6 video, which combined with AVI support, should mean that almost any file you find strewn across the internet can be played. If that wasn't enough, you've also got FLV support, so you can play back all those videos you downloaded from YouTube.
With Perian installed you can turn QuickTime X into a proper video player, while still being able to revel in the sleek, minimalist Apple user experience with tight integration into your Mac.
Perian is available for free, supports OS X 10.4.7 and higher and, if you don't want to resort to programs like VLC, could be the solution to your file playback problems.