It may look like a power adapter, but this cute device from Apple will stream music from iTunes to your stereo system. It supports
802.11b/g WiFi - Speeds upto 54 Mbps.

What's more iTunes version 7 supports concurrent streaming to multiple Airport Express, so you can play the same music, in sync, in multiple rooms.

In addition to WiFi, there is also good old Ethernet support for those with the foresight to wire up their house and the audio output is through a natty little mini jack that supports both Analog and Optical Digital (TOSLINK) output (though note the appropriate cables are not supplied with the Airport Express).
Now all you need is a way to control iTunes remotely and you're done... HeHe