After doing the sound thing for years, I finally got it together to start carrying a gig bag. I still can't afford two copies of every tool I commonly use, but it is getting close.
I found some very nice, durable 16" x 12" x 8" shoulder satchels at a local dollar store. The bag stays packed, in the closet, so I can just grab it on the way out the door. It travels with me to almost every theater gig now -- the only exceptions is when I intend to walk for a distance, and repack for a smaller and lighter set of tools.
The front pocket contains the following tools:
White "board tape" and black Sharpie marker (to mark sound boards and cables).
Black Gaffer's tape.
"Nexcare" micropore surgical tape (for taping wireless mics to actors.)
A "Minty-Boost" AA-powered USB power supply (to power some of my gadgets and charge iPods).
Small VOM (aka Multimeter).
Tweaker -- miniature screwdriver for adjusting the gain pot on wireless mics.
Earpods in a case (for all those times you forget the headphones).
Voltage Sniffer aka "Chicken Stick" (for seeing if there is power getting in).
Energizer headlamp (one of the few brands that has both red and white LEDs)
Goo-Gone, and a rag (for removing tape residue from mics and cables).
Caig De-oxit, aka "tuner spray" (for cleaning dirty connectors).
A notebook and pen.
In the flap pocket, also always there, are personal supplies:
There are more tools in the bag itself, but that is a ever-changing collection, depending on the gig in question. Often found in the gig bag;
Cable Tester (I have a nice one to check XLR, 1/4", MIDI, and other common cables)
Earclip Bag; a small mic bag holding coat hanger wire and heat-shrink tube for making earclips for wireless mics).
A couple of microphones.
A spare piece or two of mic cable (since you can never trust the house cable).
Some device to put out a MIDI pulse (a mini-keyboard, or one of my DIY MIDI gadgets).
Multi-band Scanner (good for checking for clear air for wireless, better for tracking down lost transmitters, also nice as a simple source of on-stage sound to trace a mic...plus you can listen to the radio while replacing batteries and wiping down wireless mics.
Computer (when not in its laptop bag).
And then, in my pockets always travel a few basics as well;
A hand-cranked Radio Shack mini-flashlight, converted to blue LEDs
A multi-tool (knife, screwdriver, crescent wrench) I found in one of those bins at the counter of my favorite hardware store. I have actually hung and focused an entire show (err, that's lighting talk, not sound talk) with the thing.
And nail clippers I bought in Japan. Not because it comes up often in theater, but rock climbing tends to leave you with a lot of annoying broken and ragged nails.