Squeezebox. Do a search and you'll find all sorts of information on the Squeezebox, as well as other options.
You can also use a Linkys Wireless G Music Bridge which I have seen for as little as $79 bucks. It works with pretty much any media player. The nice other nice thing about it is that it has the capability to do surround sound since it has a Coax and Optical digital out in addition to the regular L/R RCA out. Only thing is it uses WEP standard versus WPA encryption.