The developers of XBMC Media Center have released an application that can be ported on to the second generation Apple TV, the iPad and the iPhone 4, which will allow the devices to play media that is not in the required iTunes format, essentially freeing them from reliance on iTunes media types. The XBMC software requires a jailbroken device in order to be installed and is available now.
The second generation Apple TV (ATV2) has been a massive hit but is limited by the fact that it can only play media that is in an iTunes compatible format. Being that both are produced by Apple this is fair enough but will require users who tend to use other file formats such as AVI to do a lot of transcoding in order that they may stream media to their Apple TV.
The XBMC Media Center has a wide range of codecs that can be used to play media, the only caveat being that you have to through the process of jailbreaking the ATV2 device, which of course you do at your own risk. The software is unsurprisingly not available on the App Store but can be downloaded and installed using the instructions on the XBMC website.
Other options for streaming media to the Apple TV from your Mac have appeared, such as AirFlick by developer Erica Sadun which can transcode media on the fly but it still very much in the early stages. This option does still require transcoding before watching but does not require jailbreaking.
Perhaps the most impressive demonstration of the software and the Apple hardware is the fact that both the iPad and ATV2 can support 1080p content with it installed.
As always, be aware of the procedures and risks before the jailbreak.