Some knots can be extremely useful. This is one of the best I've discovered so far:
It has many uses -- securing items to the top of a rack; guying out a tent, fly, or tarp; securing a bear bag; and securing a bike to a rail or to other supports, on a ferry or train, etc., among others.
It also has a perk that is not apparent at first glance. It gives you very effective leverage (a 3:1 mechanical advantage), so you can really tighten things down. No wonder truckers like it and use it a lot. They are usually using it to secure loads, and to cinch the loads down very tightly, securely, and dependably. These same qualities can be useful while touring.
These knots are also extremely easy to tie and 'untie' -- in fact, they are so easy that 'untie' is not the right word. They just pop off when you pull. And the loop disappears immediately when you pull.
They are also easy to tie and learn. Once you've practiced a few times, it gets easier and easier until it is second nature.
There is also a series of additional knot videos that appear as thumbnails underneath,
If anyone has any other useful knots or ideas on the topic, please feel free to post.