Shairport Sync
Shairport Sync is an w:AirPlay audio player — it plays audio streamed from iTunes, iOS devices and third-party AirPlay sources such as ForkedDaapd and others. Audio played by a Shairport Sync-powered device stays synchronised with the source and hence with similar devices playing the same source. In this way, synchronised multi-room audio is possible without difficulty. (Hence the name Shairport Sync, BTW.)
Shairport Sync does not support AirPlay video or photo streaming.
Shairport Sync is a fork of the original Shairport which was based on reverse-engineering Apple's key used in its AirPort Express. Be advised that this functionality may be removed at Apple's discretion.
Installation
Install the shairport-sync package.
Configuration
The configuration file can be found at /etc/shairport-sync.conf. It contains useful comments and configuration hints. More documentation is available in the README file.