What software alternatives support streaming via AirPlay?
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.