The Highwayman's Hitch, also called the Fireman's Hitch, is ideal for tying a rope to an object when you need a quick release. The knot will take any amount of strain on the standing part, and can be untied with a very light tug of the free end.
To Tie a Highwayman's Hitch:
Pass a bight (a doubled rope) around the object
Take the mainpart of the rope and push a bight through the first bight
Take the endof the rope and push a bight through the second bight