You should know about iRATE radio.
iRATE radio is a collaborative filtering system for Windows, Linux and Mac OS X that downloads and plays music. As you rate music it automatically finds more free music that you’ll like by finding people with similar music tastes.
The software is GPL, and it indexes and filters “free” music (Creative Commons, etc.).
You should also know about the Internet Archive’s “audio” section. I particularly like their 78rpm collection – nothing more authentic than folk music straight from the hills and hollows.