The Taut Line
A taut line, also know as two half-hitches with an extra turn, is used to tighten a line, and is ideal for clothes lines, tents and the like. This would have to rank up there as one of the most useful knots to learn!
Knot Video: How to Tie the Taut Line Knot
Knot Tying Instructions
Step 1 - Wrap your rope around a pole (branch, tree trunk, fence post...), cross the shorter end underneath the long running length of the rope, and drop it down through the hole between where the rope crosses and the pole, as if you are going to tie a half hitch.
Step 2 - Bring the rope around and drop it through again (this is the "extra turn").
Step 3 - Bring the end of the rope to the front of the two loops you just made, and tie another half-hitch (just like the first one in step one). Pull to tighten.
Step 4 - You can now slide the knot along the main length of rope to tighten or loosen the line, as needed.