Cable testers – A Horizon Cable Detective handles the majority of my cable testing, while Rat SoundTools and a LAN/USB I found online comes in handy for cables that are already run.
AC outlet testers – Both a plug-in style from Ideal as well as a Greenlee GT-11 “tic trace” do the job.
Circuit breaker finder (a.k.a., “fox and hound”) – I own models from Klein Tools and Sperry, and both work great. A transmitter plugs into a standard outlet, and a receiver identifies the circuit breaker the outlet is connected to.
Going the distance – 25- and 100-foot compact tape measures help lay out a stage, and my Fluke 415D laser distance meter is handy for checking ceiling and trim heights.
Battery tester – A dedicated tester is convenient, especially when you need to check a lot of batteries. I like the compact version found at Radio Shack.
Iso, splits & DI – These include aWhirlwind line balancer/splitter, IMP DI, and 1-1 transformer barrels that can get rid of hum and buzz when interfacing with a house system or other production.
Adapters & cables – No toolkit is complete without adapters. I carry XLR, TRS, RCA, and (Neutrik) Speakon adapters and barrels, as well as a few Cat-5 barrels. I also bring a few RCA and 1/8-in TRS cables as well as IEC power cords.
Hand tools – Full-size and stubby screwdrivers, jewelers screwdrivers, wire strippers, pliers, diagonal cutters, adjustable wrench, hex keys, socket set, mini hacksaw, small hammer, and level.
Soldering kit – A basic iron, solder, and a few clamp-on heat sinks for quick field repairs.
Oil & lube – Something always needs cleaning or greasing. Enter Deoxit, WD40, Goo Gone, and 3-1 oil.
Fuses – A high-priced electronic unit is useless if it blows a fuse.
Batteries – AA, AAA and 9V, and I also bring AA NH rechargeable batteries that a Rayovac compact charger refills.