Whether you want to tie your boat to the dock so it won't slip away into the lake (again), or your lover is feeling particularly frisky, you need to know your knots. These websites should serve all your knot needs neatly.
Animated Knots alone covers nearly all conceivable knots for all non-frisky situations, including boating, climbing, fishing, and even decorative knots. (In the decorative knot section, you can learn to tie a monkey fist knot!)
As for frisky situations, specifically bondage, you might be using some of the very same knots, but since you're using them on a person, you can't just apply them willy-nilly, or someone might get hurt in a very delicate place. So be sure to learn to tie your partner safely.
While Animated Knots already covers how to tie a tie, this site covers the same knots taught to you by a cheerful looking fellow named Hendrik:
Finally, this won't teach you how to tie any knots, but it's a song called "Knots," and it's a delightful pop gem (like all of his music is):
Now, get out there and learn why a carrick bend is a useful, underrated knot.