iOS (jailbroken): AirFloat is a jailbreak app that turns your iOS device into an AirPlay receiver. It allows you to wirelessly stream music to it from anywhere.
AirFloat is incredibly simple to use. Just launch the app and your iOS device turns into an AirPlay receiver that can recieve music from iTunes, another iPhone or anything else that supports AirPlay.
If you have an old iOS device not getting much use, AirFloat is a free app that works right out of the box. AirFloat only handles audio, but if you're looking for something with a little more power, AirServer is a $US4.99 app that handles video and photos as well.