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Farrimond Friction Hitch is a favorite of mine right now. It makes an adjustable sized loop that is very useful for tensioning lines. It's very similar to a prussik. Easier to tie and untie than a rolling hitch.
Other favorites:
Siberian hitch for fixing a line to a point with a quick release.
Constrictor knot for bundling.
Trucker's hitch is great if you need mechanical advantage to produce a very tight line.
Easy to tie classics that you'll find yourself using a lot: Bowline, clove hitch, sheet bend.
I also highly recommend the app KnotsGuide., which is available for both android and iphone.