There are quite a few of these, one of the best is the squeezebox, http://www.slimdevices.com/welcome1.html?gclid =CMr-y8nu4okCFTU7MAodMQ7pFg
if your pc is on, this will find the music on it and you can browse and play through your stereo.
(Also lets you use a psp as a remote control so you can view artwork etc should you wish. )
Obviously a wireless router helps but most of the devices like this can be wired ito the network.