Knots

One of those things that all outdoorsmen (and by that I mean the women are included as well.) should know is knots. Just like tools, there is a knot for the job. And when you use the right knot, the job becomes easier. Take rock climbing, for example. While I don't climb very much any more, I have in the past. The ubiquitous knot in rock climbing is the figure 8 knot. You could use a simple overhand in some of the situations where a figure 8 is used, but a "8" is a better choice because it can be "broken" after it has been loaded and more easily untied.

With that in mind, my list of "must know" knots:
#1: Overhand. The most basic knot. The building block for most knots.
#2: The square or reef knot. Strong, but still basic. Lots of applications.
#3: Bowline. Pronounced "beau-lin" if you want to sound salty. Again, loads of applications.
#4: Prussik. Anything from self recovery to tightening your tarp setup.
#5: The other basic knot, the girth hitch. Most hitches are a variation of this single knot, but this will get you through much more than an overhand will.
And #6: Monkey's fist. Just because it is cool.

Check out Grog's animated knots for some excellent tutorials on various knots.