Perian is a FREE QuickTime plugin that greatly expands on the formats that QuickTime can play.
For me, the coolest thing is the ability to play Flash Video (FLV) files through QuickTime. I tend to save a lot of videos I find on YouTube, and until now I’ve had to either convert them to another format, or use FLVThing to play them. With Perian, QuickTime can play those files directly.
Check out the site for the complete list of formats Perian can play. I didn’t see installation instructions on the site, but all you have to do is copy the plugin to /Library/QuickTime or your-user-folder/Library/QuickTime. You may need to create the folder if you decide to place it in your user folder.
For some reason, the download links on the Perian site aren’t working, but you can get the plugin over at Cool OSX Apps.
UPDATE (11/05/2009): QuickTime X (part of 10.6 Snow Leopard) seems to play flv files without the aid of Perian. So, unless you need Perian for another format you should be able to uninstall it. Thanks Lee!