Over the past few years, my iPhone and iPad have become indispensable tools. Here's some of the apps I find useful.
Artemis Director's Viewfinder - Not the same as looking though the lens but very useful nonetheless. I use it extensively on tech scouts and a fair amount on set.
pCAM - My go to camera reference and calculation app. It's got everything. One day I was trying to plan an underwater shot with a housing that had a flat front port. I needed to know what the magnification factor would be. I wondered if it pCAM had that. It did.
Helios - Comprehensive sun position software.
MovieSlate - As long as I'm not shooting day exteriors, I always have a slate and an insert slate in a pinch. I primarily use it to take camera notes because it has several good export options.
ColorWheel HD - Great tool for talking about color with directors and production designers and for setting up color palettes.
myPANTONE - Another nice tool for collaborating with regards to color.
Panascout - Not the greatest app, but I still use it for sentimental reasons.
MapCamera - Nice when running around town looking at lots of different spots.
MagicPlan - Good for measuring a room when I don't have a measuring tape handy.
SunSeeker - Sun position software - I prefer Helios, but SunSeeker is cheaper.
KnotsGuide - Fun with rope.
TechScout Pro - Can be used to generate lighting lists. Fun to browse.
Kodak Cinema Tools
Moviola Camera Guide - Lots of good reference info and can also be used to generate camera equipment lists.
Toland - I don't use this app much, but it's pretty decent for taking camera notes and calculating exposure, filter factors, DoF, etc.
Cine Meter - While I don't rely on it's accuracy, I find the display easy to use when I need to discuss exposure with a director or production designer while walking around set.
MovieLooks & VideoGrade - a couple apps that can do some basic color grading and video filters. Useful for generating a quick & dirty look when the director asks something like "What would this look like with a bleach bypass?"
Shot Designer - Useful for generating overheads.
BizXpensTrkr - I use it for logging mileage and expenses. I don't love it but haven't found a better one yet.
Dropbox - I put .pdf camera manuals there for easy access along with visual reference materials, storyboards and scripts related to current projects.
Videos - It's nice to always have my demo reel with me.
GarageBand - Good if one needs a click track for a dance sequence. I've also used it to drive a subwoofer covered with a swimming cap to launch paint and powders into the air for some high-speed work. That's fun because rather than saying "roll camera" we say "drop that bass."