Taut Line Hitch
We find that one of the most useful (more obscure) knots is the Taut Line Hitch. We use it for tying down loads in our trailer (and when folding it) and we even use it to help support items that are being lifted about head height into place.
This knot can be tightened after it is already tied an in use which makes it ideal for use at the end of a rope under tension.
Wrap the rope around an end point (if desired) and place the loose end of the rope below the rest of the rope.
Wrap the end around the rope into the loop twice.
Bring the end of the rope behind.