Airfoil has just been updated to version 5.5 and introduced support for Chromecast on OSX (Windows coming soon). There is a 20% off coupon in the latest newsletter. Airfoil is fantastic software, one of my most used utilities and well worth the cost. It allows you to send any audio stream to any combination of airplay speakers, bluetooth speakers or even other computers.
"Use Airfoil to send audio from your computer to outputs anywhere on your network. Hear your music through thousands of devices, like the Apple TV, AirPort Express, and other AirPlay devices, as well as Bluetooth speakers, iOS devices (iPhone, iPad, & iPod Touch) and even other Macs and PCs!"
StreamWhatYouHear is a free alternative for Windows, though I doubt it is as capable as Airfoil, as it's free. Basically it can send the output of the computer to another device on the network.