My exposure to non-mainstream music is sorely lacking. This is all I have to counter with:
"Mainstream" is so relative as a term, though. There are people I know who consider Nightwish to be mainstream, despite them never having had anything in the UK chart to my knowledge and so don't fit any meaningful definition of "mainstream".
I think once we understand how "mainstream" and "alternative" music works, we'll understand everything about the human condition. I could re-write all of skepticism and human rationality with this as the analogy.
Here's the most obscure group I know, though I'm not sure if local unsigned group featuring a guy I vaguely knew at school counts as "obscure" as it's practically unknown by definition. But the EP is free to download, apparently: http://esolovesyou.bandcamp.com/