Last.fm
I also found these two EPIC threads:
http://forums.questionablecontent.ne...c,19792.0.html
http://forums.questionablecontent.ne...c,25859.0.html
I also will search by genre on google, things like: "Underrated black metal bands" or something like that.
Reading reviews is a good way to get pointed in the direction of new music. I go to Sputnik Music. I read Pitchfork every week or two.
The radio and TV. I find myself listening to the radio in my car and I constantly mine lyrics to google for later when I hear a song on TV.
At this point I have experienced so much music that I largely will search for new bands based on my current mood. For example, if I am listening to some Dub Step or IDM or Techno in my collection I'll google around for similar or new bands.
But definitely check out those two threads, they are full of SHITLOADS of amazing music. Epic stash.