Windows/Mac and iOS: Apple's AirPort Express has long been an awesome way to stream music from your PC across the house, but if you don't have one, iOS app WiFi2HiFi lets you stream any audio to your stereo using your iOS device.
While you could always hook your iPod or iPhone up to your home stereo and play music directly, you may not store all your music on your iPod. Also, you can't always play other audio streams — like Grooveshark — on your iPhone. With WiFi2HiFi, however, you can stream any audio from your computer to an iOS device on the same Wi-Fi network. All you need to do is download the app on iOS and install the small desktop app on your Windows or Mac computer — that's it. Whenever you start it up on your computer, it will automatically detect your iOS device with the app running and stream all your computer's audio to it.
WiFi2HiFi is a $1.19 download for iOS devices. Its Windows and Mac counterparts are free. Quite a far cry from the $149 AirPort Express.