Mac: You can stream music on your Mac wirelesly through AirPlay speakers, but it's limited to just one device at a time. Airwaves is a handy app that lets you broadcast your audio to multiple AirPlay speakers all over your home, kind of like a poor-man's Sonos.
With AirPlay, you're often limited by whether or not the music or movie player you're using supports it, and even then you can only select one AirPlay target at a time. Airwaves gets around the problem by giving you a system-wide control that switches from your Mac's speakers to any AirPlay-compatible device on your home network. If you want to broadcast to multiple rooms and multiple devices, Airwaves can handle that with no issue. The app is available now for Macs running OS X 10.8+ and will set you back $4.49 in the Mac App Store.