It's like a really shit movie plot—a developer has cracked Apple's code for Airport Express and published an emulator online for all to see (and grab), which emulates the wireless networker. That means you can use Airplay to stream music to other computers, games consoles...
The emulator, called Shairport, can trick iTunes and iOS devices into streaming the music to other devices, foregoing the need for Airport Express. As a commenter on Hacker News explained, the process works like this:
Previously you could do this:
iTunes — stream to —> Apple Airport Express
3rd party software — stream to —> Apple Airport Express
Now you can do this:
iTunes — stream to —> 3rd party software/hardware