The Velvet Underground is arguably the quintessential highly influential band that was ignored by the mainstream during their relative hey day.
Reed was an iconoclast, blowing up both the bubblegum pop-rock and the hippie-driven "Summer of Love" warm fuzzies and anti-war screeds with tales of druggies, street toughs, weirdoes and freaks, the folks with whom most rock songwriters at the time weren't bothering.
R.I.P. Lou Reed
For those unaware of Reed and his music and legacy, Here's what some of the fuss