The PPU formed in 1968 and lasted two more decades, skirmishing with and occasionally being arrested by police and security forces. They gave heart and happiness to dissidents in Czechoslovakia and throughout Central Europe, including Poland's Pomarańczowa Alternatywa (Orange Alternative), which staged "happenings" throughout Poland during Martial Law in the 1980s.
Of the PPU, Havel said, "We may have our freedom now, but no one would have it without the inspiration of the music of the Plastic People." Can you imagine Obama or Bush (not to mention Romney or Gingrich) saying something this about any artist--and meaning it?
You can see a short history of PPU and hear just a bit of their music on this very short YouTube video: