Beathound (http://beathound.com) is making between $50 and $100 a month in affiliate revenue from the various music stores. I'm working on rolling out some new features that will increase that.
Interesting! Plenty of room to grow considering it's iTunes-only and (anectodally) most people I know don't use iTunes... What music stores do you support? I've seen lastfm has 7digital, amazon, itunes and juno but I've never used any of them.
Right now it's iTunes only, but I'm doing a lot of research into what to support next.
Some of that research has led to new projects (http://magic-playlist.com puts music recommendations into your Rdio or Last.fm account), but all of them contribute to the overall ecosystem. I'm hoping to eventually support everything.