After releasing a jailbreak app which enables AirPlay functionality via third-party apps, and then AirPlayer which allows you to AirPlay videos on your Mac from your iPhone as if it was an Apple TV, Erica Sadun has now released AirFlick which basically allows you to stream just about anything on the Apple TV from your computer running Mac OS X.
In a blog post at TUAW, she explains AirFlick as:
AirFlick offers the potential of real-time transcoding of otherwise unsupported file types into Apple TV-compatible data. It also allows you to open videos located on the Internet by pasting a URL and clicking the play button. I was able to watch a number of Internet Archive (archive.org) mp4 videos on a big screen TV by browsing that website, selecting URLs, and opening them with AirFlick.
AirFlick works by transforming your Mac into a web browser, the same way AirPlay works on your iPhone or other iOS device. For Mac-based files, AirFlick tells your Apple TV to connect to a local URL and serves the data that the Apple TV plays back. So long as those files are in a supported format such as mp4, m4v, mp3, etc, the Apple TV can read and display the file data.
Here is a short video of it in action:
Download AirFlick for Mac