If you own a Macintosh and an Apple TV, I have created a new tool that allows you to stream data outside of iTunes. It's called AirFlick, and it's now available in alpha form for download and testing.
Last week, I put together a related application called AirPlayer that allows you to stream video from your iPad using Apple's built in AirPlay services and demonstrated it on TUAW. AirPlayer works by emulating an Apple TV using your Mac's built in Bonjour networking capabilities. Below is a description and brief video explaining how it works.
Update: TUAW reader BC adds:
How to Add Live Conversion to Erica Sadun's AirFlick App
Ok, by doing this hack, you can stream ANY video format sitting on your Mac to your AppleTV. I'm testing out an mkv file right now, and it works like a charm!
One extra little note: you don't have to necessarily download the unofficial Mac AirVideo client above. You can instead begin playback of a video from within the AirVideo iphone app, then go to the mac serving up the stream, and type "ps ax | grep ffmpeg", and grab the alphanumeric string following the --conversion-id flag.
Then paste into AirFlick the following: