What software alternatives support streaming via AirPlay? The Apple TV arrived! I'm slightly bummed that I can only stream from iTunes, and therefore only use formats that iTunes supports. I haven't found a single other video player that supports AirPlay, though to be fair, AirPlay has only really been around since the new Apple TVs started arriving.

What other software supports streaming audio and video to AirPlay devices? If none, is AirPlay one of those closed Apple standards or have other projects pledged to support it?
To date, there are no 3rd party software implementations of AirPlay. The spec itself is fully patented (though most of these patents relate to the hardware aspects of the system) and is essentially a proprietary standard unique to Apple (though several hardware vendors have announced hardware that will use the standard - XtremeMac, Denon). The current analysis of the protocol itself seems to indicate that it is a further evolution of the previous AirTunes protocol (which itself was built on top of Bonjour), so you are likely to see software implementations appear after developers have had more time to experiment with the protocol.