Have you ever tried to tie something down for transporting, but just couldn't get the lines tight and/or during transport the lines would continually loosen? Then this is the knot for you! I learned this knot back in the 70s when specialty car racks and ratchet straps were rare or unheard of. I initially used it to tie a canoe on a car rack, both attaching to the rack as well as the lines to the bow and stern of the canoe. Even with all the new gizmos available today, this knot still shines because all you need is a rope and ropes don't hum in the wind like straps.
How to Tie a Trucker's Hitch Knot — eHow.com
Animated Knots by Grog: Slight variation on the two examples given above.