Inspired by a recent AP article, I have renewed my intention to refine my knot competence. (Despite having somehow, and worryingly, been taught to tie a bowline behind my back while treading water, I have effectively retained none of it.) They recommend IWillKnot for a very basic overview of knot tying. Super simple, but definitely a solid starting point. For the more advanced knottists, try Animated Knots by Grog, which is broken down into genre of knot. Read up!