Picture this: you've set aside four or five hours to catalog your baseball card/stamp/wine bottle/My Little Pony collection, and would like some television to entertain you. Your Special Collection Sorting Table isn't anywhere near a TV - or an outlet. You'd like to catch up on ABC's Castle while you're doing it, but don't necessarily need to see the TV (Stana Katic notwithstanding) because you'll be cataloging. And you don't have Hulu Plus. Then you need SoundWire, an app that lets you stream audio - any audio - from your PC to your Android phone.
SoundWire works sort of like a remote desktop viewer in reverse. Install the server application on your Windows PC (sorry Mac folks) then connect it to the app on your phone over your home's WiFi network. After a brief authentication, all your PC's audio from any application will be broadcast through your phone's speaker, headphone jack or Bluetooth connection. It'll even mute the volume when you get a call. That's just downright convenient.
The free version of the app includes an audio nag every half-hour, but the $4 paid version is distraction-free. Both apps work with all recent versions of Android, and can connect to multiple PCs for different streams of music. Technically you could use it over a wireless broadband connection as well, but you'll have to know your way around a little port forwarding and keep an eye on your data usage if it's limited.