Tangle Free Rope Storage
Well, it's not a knot, but if you're using rope there's a good chance you'll be interested in this technique for keeping your rope tangle free.

Siberian Hitch
This is the #1 knot I use when hanging the tarp. I probably use this knot more than any other. 
It's very fast to tie, can be tied with gloves or even mitts on, it holds strong with tension and can be undone with a tug.

Rolling Hitch (aka Taut-Line Hitch)
Great knot for tents and tarps.  It's a slip knot that will allow you to tighten or loosen a line but holds fast under tension.