Mainly surplus Swedish army packs with external frames, I have both the smaller ~25l as well as the bigger ~35l aircrew versions. They're made of heavy canvas,

Mainly surplus Swedish army packs with external frames, I have both the smaller ~25l as well as the bigger ~35l aircrew versions. They're made of heavy canvas, have a strong external but integrated (the top of the frame is held in a leather sleeve on the bag) frame and are comfortable to carry even with heavier loads. They can be found at military surplus stores for around €15.