In reply to Orgsm:
> Originally used to secure Zeppelins at ports.
Sadly there is no actual evidence of this
Since people seem to be posting serious knots now, the Alpine Butterfly. Useful for so many things, and usually easy to untie to! Examples (off the top of my head): tying the end of a sling in a sling belay, making a loop in anchor ropes to hang a guide plate from (tie in with the rope AND guide mode!), making a large loop to make a Y to clove hitch to a second belay stake/second piece of gear when building an abseil anchor, joining abseil ropes together when running on two strands and retrieval is not required, clipping someone in the middle of a rope, isolating a 'bad' section of rope, the list goes on...