for a quick reply
* Multitool - Screwdrivers, pliers, knife
* Tweaker - small flat screwdriver for taking apart XLR connectors, adjusting gain on wireless mics & some other gear. Also referred to as a "greenie".
* Flashlight - Flashlight apps on your phone burn through the battery in no time, and you might need that phone to save your butt.
* Basic Adapters: 1/4" to RCA, RCA barrels, 1/4" barrels, XLR barrels (both ways), XLR Phase Reverse Adapter (for snare up & down), XLR Audio Pads (to prevent overloading your board even at minimum gain), XLR Ground Lift Adapter.
* Cables: stereo 1/8" mini to RCA (computer audio/mp3 players), 1/4" to 1/4", Y cables of all kinds, T/R/S to XLR (both ways)
* AC Cube Tap - To get you more outlets fast
* AC Ground lifter - A less than ideal, but quick way to stop a ground loop hum.
* Multimeter - read AC wall voltage, check for shorts or open wires, test cables, test battery voltages.
* Extra Direct Boxes - to provide isolated feed to video, house sound systems, etc. Transformers are indeed your friend.
* Deoxit(tm) - Cleans oxidation off of contacts that haven't been used in a while. Oxidation you can't even see can be enough to stop signal flow and leave you high & dry.
* Butterfly mic clip - to accommodate any size mic you might have to deal with in a hurry, such as handheld wireless mics.
* Spare Batteries - For you & possibly others. Great way to make a friend for life.
* Spare IEC Power Cords - The detachable 3 prong types found all over the place in audio. There's a way to bring setup to a halt instantly.
* Spectrum Analyzer App - The one I use on my phone does 1/6 octave, plus it has all sorts of test tones & pink/white noise. Bear in mind the internal mic does have a low end cutoff, but it is entirely usable.
* Small subsnakes - Great for when you're running low on regular mic cables, plus they reduce stage clutter too. The ones I have around are 5-6 inputs & about 20 feet long.
* 3 pair mini snake - This is about the size of a regular mic cable, but with 3 XLR plugs at each end. Great for last minute stereo L/R feed requests. Only 1 cable to run, and it comes with a built-in spare.
* Single Channel Mic Splitter - You just never know when you might need to tap into an audio signal. The ones made by Whirlwind are great.