I tried building an primitive version of that idea a few years ago, called Downstream [1] that scoured MP3 spider sites for URLs, then fed them into AxWMPLib (MS's drop-in media player).
It had a few ideas that were before their time, like a playlist DJ, and what a 17 year old me thought passed for a 'stylish UI' [2]
There used to be HipHop [0], a simple app that scraped mp3 sites and streamed music from them. Although it was fully functional (and still appears to work on my side), there is no music discovery, only search - which is a limiting factor for this app. Previous discussion on hn [1].
It had a few ideas that were before their time, like a playlist DJ, and what a 17 year old me thought passed for a 'stylish UI' [2]
[1] https://github.com/MattRyder/Downstream
[2] http://screenshots.en.sftcdn.net/en/scrn/328000/328491/downs...