I can't track down the name of this knot, but for ropes that will be under tension the knot I learned when working line service in high school for tying down aircraft has been tremendously useful. [0] is an excellent example of how to tie this knot, it holds tremendous tension while being easy to release. I've found this very useful for camping and general securing of loads.

If anyone knows the 'proper' name for this knot I would greatly appreciate it.

[0] https://www.youtube.com/watch?v=UMITtV80xpY