I second this. I'm using squeeze server on a linux box with all my mp3s on it, slaved to a bunch of old Dell Rio Receivers that aren't fantastic, but work good enough for squeeze. They're all in the basement (I've actually got in-ceiling speakers throughout the house wired to the server room) and controlled through browsers and an iPhone app.
In most cases, though, just stick those in different rooms (find 'em on eBay, maybe) or a bunch of Squeezebox players (slimp3, etc.), hook them up with ethernet, and you're good. Use the browser, or an iPhone or iPod touch for controlling them, or you can even buy a fairly cool gui controller from Logitech that works with the whole thing. (though I'd probably recommend the iPod Touch route, 'cause you can do a lot more with it and it costs about the same as the controller does).
Anyway, it's cool, and reasonably open too (their hardware information is even available on a wiki, with block diagrams and software source code).