There are a vast number of knots that are useful, some even life saving in a survival situation. I have few videos demonstrating such knots, but I have covered two of the most important:
The Bowline
This is almost certainly the most important knot to have in your skill set. It is quick to tie, will not slip, and easily comes undone after use. The uses are quite literally limitless. More in the video:
The Taut Line Hitch
This knot is very useful for adjusting tie downs quickly and easily. For example, a tarp could be held down by a series of these knots and be made very tight so the wind cannot make it rise, and easily be removed simply by sliding the knots later. A taut line knot is also used to keep large tents from sagging by keeping tension on the lines leading to the stakes. A slight variation of this knot is used to climb straight up a rope when doing tree trimming. I currently use two of these knots to hold down a blanket as a seat cover on my truck. As with any good knot, a taut line can easily be untied when desired.