The resourceful people from AwkwardTV have released ATVFiles, a simple browser plugin for the Apple TV Finder. Like XBMC, this plugin can play video files without transcoding and syncing/streaming them. As for the codecs, you'll have to get them yourself as ATVFiles does not provide them. AwkwardTV recommends Perian and the A52Codec.

Since the downlaod doesn't include a readme file, here are the installation and usage instructions as well as the known issues for the plugin:

Installation

1. Copy ATVFiles.frappliance to this folder: /System/Library/CoreServices/Finder.app/Contents/PlugIns on the Apple TV. You may want to create a symlink from there to some more convenient place.
2. Restart Finder. The command killall Finder will kill it, and it will be automatically restarted.

Usage

1. Enter the new "Files" menu on the main menu.
2. It lists files in /Users/frontrow/Movies, so mount or put your video files in that folder.

Known Issues

* AFP mounts don't seem to show up if they're in a location symlinked to. Mount them directly under Movies instead.
* Non-video files are played back using the QT player.
* No previews, etc.
* The icon is ugly and scaled way up.
* A NSGenericException: backtracing is raised during Finder startup. This is normal.
* The plugin isn't localized, and shows up as CFBundleName in the main menu for languages other than English.