Last.fm has an exceptionally good music discovery engine and is pretty much the source of how I hear about new bands, keep up with favorites and land new albums and recommendations from my friends and neighbors.
Pandora is also pretty rad for getting great new recommendations based on what you already listen to, although it's much more interactive than the tag system on Last.fm.
And for those of you who don't know,
we have a tightly knit conclave of music junkies here on GFF as well, so feel free to browse that thread or join in on the recommendations. Some of the best music I've found, I've found through my boys (and girls) down in M.E.C.
Jam it back in, in the dark.