i can tie half hitches and bowlines and clove hitches and larksheads, but i got out my knot book and picked three i didn't know.
i learned a proper tautline hitch, and because nearly every source suggests that a much more USEFUL knot for the same purposes would be the midshipman's hitch, i learned that, too, because it seemed like the right thing to do. it is mostly recommended for tying down tent stakes, and because the midshipman's hitch is made up of an awning knot and a half hitch, it is apparently recommended in the case that you fall overboard and just HAPPEN to be able to catch a trailing line.
i also learned the butterfly loop (also called the lineman's loop), which has practical applications and is a good knot anywhere you need a loop with even load on both sides, and it can be used to isolate a bad section of rope in a pinch.
|butterfly loop (closeup of knot)|
|good luck knot|
|closeup of good luck knot|