[jvm_]

/r/listentothis is good for finding different genres, and usually a good song from that category as well.

[ulrikrasmussen]

And /r/ifyoulikeblank for querying the hivemind for suggestions for other stuff that you might like.

[at_a_remove]

This has been a great site for exploring various microgenres.

I forget which service offers this -- is it Pandora? Sometimes I have some pretty hyper-specific requests that I have a hard time describing, and most of the "if you liked X you will probably like Y" are entirely too clunky for it.

[ddek]

If you're into most alt styles, /r/indieheads is pretty good too.

I quite like going on rym and playing whatever's on the front page, that's exposed me to many genres I'd have skipped otherwise.