Personally I'm not finding any of those apps particularly essential... A good DOF calculator can be handy but in the heat of the moment, typically, I'll just shoot at three different apertures if I'm *THAT* concerned about getting it JUST-SO in camera.
I am finding
DSLR Dashboard (link goes to Android version) to be pretty freakin' cool, if somewhat less than "essential". It turns your smartphone (if it's supported) into a full blown remote control for your camera. The app itself is free but the manual, a downloadable e-book, will run you $5. You'll most likely also need a $5 dongle to connect your phone to your camera via the accessory port.
I did it just for fun and hells bells if it doesn't work like a champ.
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