Sonos is an app for the iPad which can control any Sonos ZonePlayer with all of the features of their ZoneControllers. The app features the ability to stream millions of songs and stations all over the house, and play the music stored on your computer, NAS drive or Apple Time Capsule even when the computer is off. It can play the same song in every room or different songs in every room; control the music selection and volume in each room individually.
The app also searches for any song with the touch-screen keyboard, drag and drop songs or albums into the queue or now playing pane. The app is free. [link]