Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any application and send to your AirPort Express units, as well as Apple TVs, and even other Macs and PCs running Airfoil Speakers! Transmit audio from RealPlayer, Windows Media Player, QuickTime Player, and other media players. Send audio from web-based applications like Pandora, Last.fm, and other to the AirPort Express. You can even stream audio from audio devices like RadioSHARK, XM and Sirius radios, around your house.
Requirements: Intel, Mac OS X 10.6 or later.

Version 4.6.1:
A "Process Source" bug, where audio from some apps was not captured when Instant On was installed, has been corrected.
A "-1600" error, where connections to some AirPlay devices could fail, has been fixed.
The cursor is now hidden when Airfoil Video Player is in full-screen mode.
Airfoil now checks if a source app is quarantined by Mac OS X, and warns if so.