Some leftist and leftist-ish artists off the top of my head:

Refused

Dead Kennedys

Crass

Chumbawamba (yes, that band)

Discharge

Parquet Courts

Idles

The Coup

Gang of Four

CCCP Fedeli alla Linea

Minutemen

Propagandhi

Billy Bragg

Phil Ochs

Reagan Youth

Godspeed You! Black Emperor

Thee Silver Mount Zion

Rudimentary Peni

Panopticon