The only knot I use frequently in the woods is the Marlin Spike hitch. Most everything else is already tied or spliced. In preparing ropes or cords, I most often use Lark's Heads, Alpine Butterflies, Prusiks, and Double Overhands.
I've got Knot Guide for iOS. I've found it to be a really handy tool for learning knew knots and for helping out when I'm trying to remember a knot that I don't use very often.