Here are a few of my favorites in no particular order:
Dental mirror and penlight for viewing inside wall cavities.
Dental pick for grabbing wires from small openings, scratching foil bridges on circuit boards, etc.
Box of about 5 pounds of beaded chain which I bought at a garage sale for . When fishing wires from attic to basement in older homes I would often locate the wet wall (plumbing chase) where the DWV stack rises through the attic and drop one end of the chain along side the pipe. Due to its weight and flexibility, as I paid it out the chain would find its way to the basement through any available openings at each level below.
Tone set made by Progressive. Clip the tone generator's leads to any open circuit and you can trace wires behind walls from several feet away. This was great for deciphering unlabeled wiring during takeovers, completions of builder prewires, etc.
Butane soldering iron. No cord to knock over lamps and stuff. :)
Right-angle, cordless drill from Makita. When retrofitting ceiling speakers I'd cut out the opening, reach in with a shortened 3/4" paddle bit (Note to Olson: paddle bits are perfectly safe once you learn how to use a drill) and drill the top of the wall to drop my cables down.
Laptop computer for downloading panels. Great for diagnostics and reading logs on service calls as well as doing the initial configuration on new installations.
One of my suppliers carries a small tube attachment that will fit over a can of smoke. It concentrates the smoke and usually only takes a single short "blast" to set off a detector. We've significantly increased the number of detectors we can test with canned smoke as a result.
Extension rods are good for heat detectors. Some companies use the small butane soldering irons with the "heat" attachment. This eliminates the "open flame".
Each manufacturer has their own smoke detector sensitivity testers. Instead of making a huge investment (some can cost as much as $6000.00 and will only test one "family" of smokes) call some of your local "friendly" competitors. Find out which ones they have, and purchase one they don't (but need). Then you can talk about "sharing" or "pooling" the testers.
For really high smokes there are extension kits, but whether you want to get them is up to you. If you do a lot of theatres, you might want to invest in one.
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