Warthur
This is a sort of companion piece to the First Four Years; whereas that compilation is a quick port of call covering all of the band's official releases from before Henry Rollins joined the group, this spends its first three sides collecting various unreleased sessions from their pre-Rollins vocalists. The result of this that you get a lot of variant songs which you may already familiar with from First Four Years or Damaged, but it's actually kind of fascinating to hear how these songs evolved both with the shifts in vocalists and with the development of the band's early style.
Side 4, Crass Commercialism, is a sound collage made up of various radio spots advertising Black Flag shows; that's the sort of premise which looks awful written down like that, but it's actually a hoot to listen to given the sense of humour on display. Certainly, this compilation is worth grabbing if you want the whole story on the early years of the band.