[ame=http://www.youtube.com/watch?v=hQg046aoV9Y]YouTube - AirPlayer demo[/ame]
AirPlay is a significant extension of the capability of iOS devices to act as a real media player, rather than just a portable. This is a boon to people who already have an Apple TV, or don't mind dropping $99 on one. But what if you have neither an Apple TV, or a TV set to hook one up to... but you do have a nice big Mac screen.
Enter Erica Sadun's AirPlayer. The app runs a Bonjour server on a Mac which simply advertises itself as an AppleTV. Once it's up and running, all you need to do is select your Mac as a destination on your iOS 4.2 device and it's now a big Apple TV. You don't even have to jailbreak to stream video; that is, if you don't mind being stuck with YouTube. Anything with DRM on it will give you a big blank screen. Jailbroken devices running AirVideoEnabler, on the other hand, will be able to stream from any app that relies on Apple's MediaPlayer framework, including VLC, AirVideo, and HTML5 videos in Safari.
Erica's done a lot of work understanding AirPlay and Bonjour in general, and it paid off in this proof-of-concept app. It's still "extremely alpha" ware, but hopefully will inspire a lot more interesting projects with AirPlay. You can download the current version from Erica's website.