The Trucker’s Hitch (Lorry Knot in the UK and parts of Canada) allows the scout (or sailor or whatever) to easily tighten a rope, yet easily untie the knot. Its most common use is for tying loads (thus the trucker moniker) such as a canoe to a cartop, a tarp to trailer or any application where a very tight rope is needed. It provides the advantages of a block and tackle, without the heavy hardware.
To tie the knot, first make a single Figure 8 knot (red) in the middle of the rope. Loop the rope back through the Figure 8 (black line) and around whatever you will use for an anchor. Bring the end of the rope back through the first loop and pull like heck to make the whole thing tight. Finally throw a couple of half hitches (green) around both strands of the big loop to secure the knot. Even under the most severe tension, the knots remain easy to untie.

Hit Reload to play knot tying animation