5 knots that every boy scout should know (with pictures):

Square Knot.  This is also know as the Reef Knot.  It's uses include tying two ropes together, and tying two ends of a rope to make a loop.  Always exercise caution when using the reef knot, as it can't support much strain.
http://www.animatedknots.com/reef/#ScrollPoint

Bowline.  This knot is best used to create a static loop.  This will not slip.
http://www.animatedknots.com/bowline/index.php?LogoImage=LogoGrog.png&Website=www.animatedknots.com&Categ=boating#ScrollPoint

Larks Head Knot.  This is also known as the Cow Hitch.  This is good for attacking a rope to a stationary pole (or other rope).
http://www.animatedknots.com/cow/index.php?LogoImage=LogoGrog.png&Website=www.animatedknots.com#ScrollPoint

Figure 8.  This knot is very good for creating a stopper at the ed of a rope.  A double figure 8 can also be used as a loop (mostly in climbing).  Another application for the figure 8 is to add weight to the end of a rope to make it easier to throw.
http://www.animatedknots.com/fig8_/index.php?LogoImage=LogoGrog.png&Website=www.animatedknots.com&Categ=boating#ScrollPoint

Overhand Knot.  This is arguably the easiest knot to tie.  It makes a very good stopper knot.
http://www.animatedknots.com/overhand/index.php?LogoImage=LogoGrog.png&Website=www.animatedknots.com#ScrollPoint