Ex.fm

Exfm turns the entire web into your personal music library. As you browse, exfm runs in the background indexing every MP3 file you come across, building a music library for you. exfm will continue to check the sites you've visited, adding new music for you to listen to every day.

Works on
ex.fm can detect embedded music files like standard mp3 but also tracks on Bandcamp, Tumblr, Filmbay and Soundcloud pages, although that last one only when on album/track-ages and not the overview-pages.

Browser versions
ex.fm is available as a browser add-on for Safari, Chrome and Firefox. The current version for Chrome is better since it lets you add tracks separately, this not yet possible in the version for Firefox were it adds all the available tracks to your queue (not sure how it works in Safari).

Music Discovery
The website itself gives an overview of your profile, trending artists/tracks/music sites and the people you are following. There used to be an option where you could see the tracks you recently found sorted by URL but that option has been removed.

An interesting option is to use ex.fm as one of the online sources of popular music for a genre by using MusicBee.