Soundmans Checklist:

The most useful of all:

CD or MP3 player with quality recordings of music you are very familiar with, various styles. Used to calibrate sound balance of the rig. (If you use MP3s use a very high bitrate to keep the quality). Can be handy to have one 'difficult' track, the sort that sounds fine on a good system but crap on anything less.

Headphones: If you have your own headphones you immediately have a reference, no matter what. These need to be of the closed type with good isolation, a reasonobly accurate sound balance and loud.

Many of the DJ type headphones are suitable and a commonly used/available model such as the Sennheiser HD25 & HD25sp headphones can be recommended. They are something of an industry standard, are easy to get hold of and you can easily get spares. And for live sound they do the job well.

Gaffa tape: Essential, but then you probably know that already...

White insulation tape (yellow will do): For marking track inputs on mixing desk and wherever needed (removes easily, can write on it)

Indelible marker for writing on above (A ball point pen often works as well). Chinagraph pencils are traditionally used here, but markers are more available, just make sure you keep to the tape and not the desk!

Notepad and pen/pencil:  note where everything is on stage while setting up the band, then you are not guessing when you get back to the mixing desk; noting cues, settings and even announcements.

Torch: There is always a moment where you need to see in a dimly lit venue, whether just some stage notes, or hunting for that elusive connector on stage. Modern LED torches are great here.

Not totally essential, but a damn good idea:

Sound pressure level meter. It can be useful to keep an eye on the average sound pressure level in your venue, this not only trains the ear but can be invaluable where limits are being placed on maximum sound levels.

Radio Controlled Clock. If you are doing gigs that have to conform to strict time conditions one of these takes away any room for error and cannot be disputed as they are locked to the atomic time code.  

Good collection of cable adaptors inc xlr back to back and phase reverse.

The Toolbox:

Multimeter: Doesn't need to be anything fancy, cheap digital multimeters are availably everywhere these days, so there is no excuse not to really. 

Cable tester for all audio leads. 

Simple signal generator and tracer. This is a simple device that outputs a tone (often around 1kHz) via a volume control. The tracer section is a simple pre-amp that feeds an earpiece or sometimes a small speaker. Can be used to check every part of an audio chain from cables to amplifiers, mixers and effects.

Small wire cutters: electronics type, for cutting thin wires accurately.

Large wire cutters, piano wire grade. Maybe compound type. For cutting cables, trimming guitar strings and all manner of chomping and cutting...

Soldering iron and solder. Get a good one such as Antex, the X25 model is a good choice. With a range of bits, this will cover all soldering requirements from circuit boards to speaker cable. Small pocket battery or gas ones can be a handy addition as well.

Miniature pliers: Long nose and needle nosed, for gripping wires and small parts.

Small pair of sliding jaw grips. There are a couple of makes, CK and Knipex who do a small size that have a surprisingly wide maximum jaw opening. The are not cheap, but well worth it as they are compact, very tough and amazingly useful.

small pair of locking jaw grips, sometimes known as 'mole' grips. Particularly useful for holding small parts for soldering.

Screwdrivers: A selection of flat blade, philips and posidrive types in standard sizes including minature 'jewellers' set. An electric screwdriver is a great asset and the use of plug in bits removes the need to carry lots of individual screwdrivers. An electricians mains tester screwdriver should be included.

Small spanners or nut spinners: 1/4 drive sockets can be handy especially the long reach ones. can be driven by electric screwdriver.

Fuse wire: surprisingly useful in an emergency, not just as fusewire either...

Spare connectors: audio and mains. There always comes a time...

Small torch: even if you have another one, keep a spare in here.

Spare batteries; Not only for your own needs, but keep one or two 9V batteries for those artists who forget to bring their own spares. The occasional cost is worth it to avoid hassle when trying to keep a show going!