Midshipman's Hitch or Tautline Hitch
This hitch is really just an adjustable loop used for non-critical applications where you need to adjust the size of a loop to apply tension to guy lines, like on your tent.  After you tie it, grab the bulk of the knot and slide it where you'd like.  Once you release it, the loop will hold how you've sized it.  There are a few common ways to tie this knot, and all serve the purpose.  The center diagram is the form most commonly referred to as a Midshipman's Hitch.  It has the same general form as the first diagram, except that the first coil made around the standing part is choked by the second coil.

The knot must be drawn up very snugly to work, and may not work at all on especially stiff or slippery rope.
Don't expect too much from it.  It's not a very secure loop.  If you tie down a load on the back of a truck with this, you're likely to find that your lines have gone slack after a few miles.  A Versatackle is a better choice there.

Related page:  Blake's Hitch (Prohaska Hitch)