The latest release of Rogue Amoeba’s Airfoil is great. I’ve got a Mac Mini wired up to my A/V system, and with the Airfoil Speakers app running on it, I can send audio from my laptop to the good receiver via the Mini – no Airport Express necessary. Also, it will send audio to multiple devices, including the local computer, all in sync.
With an Airport Express in the home office and the Mini in the living room I’ve got the whole house wired for sound. And Airfoil streams any audio, not just iTunes. This means I get Pandora everywhere and it’s awesome.