Qualifiers here: "indie" means not tied to a big publisher or label. "Underground" would mean music that does not have a massive PR machine behind it or a massive label presence, though I did toss in some accounts that periodically talk about bigger/mainstream acts.
There are going to be some misses, of course, just by virtue of trying to come up with a comprehensive list that can sort of comprise the "pulse" of what indie music is doing these days. The community on here feels a little disjointed so I'm mostly trying to put it all in one spot.