Trucker’s Hitch
Probably the most useful hitch there is, the Trucker’s Hitch allows a line to be pulled tight as a guitar string and secured. It is used by truckers to secure heavy loads in place and works equally well tying canoes and other objects to the tops of cars. Once the line is pulled to the desired tension using the pulley effect of the loop in the middle of the line, the knot is secured with a couple half hitches around one or both lines.

Note that the animation below doesn’t show the pulling of the line tight - you can really pull hard to tighten down the rope onto your load.