A collection of cables and adapters to deal with any situation is handy. The following would be a good start:

1 x 30m Cat5 cable

1 x 1m XLR-XLR cable

2 x 10m XLR cable

2 x XLR Female-Female barrel

2 x XLR Male-Male barrel

1 x 3.5mm stereo jack - 2 phono plugs

1 x 3.5mm stereo jack - 2 XLR Male adapter

10 x phono plug-1/4" jack adapter

10 x 3.5mm stereo jack-1/4" stereo jack headphone adapters (expect to lose 1 every show!)

2 x 3m MIDI cables

1 x 5m MIDI extension cable

USB cables, 6pin-6pin Firewire cables, 4pin-6-pin Firewire cables.

Mains and modem adapters for work abroad.

Tools

Credit card - You don't want to run out of money mid-way through a production week, or on tour.

Maglite & spare bulbs - I have Neutral Density gel in mine so it can be used during a performance without attracting the attention of the audience.

Insulated screwdrivers, snips and pliers

An infinite supply of fine-point Sharpies & White PVC tape

Various test boxes are useful for troubleshooting:

Whirlwind Q Box for testing audio lines

Behringer make a fairly useful and inexpensive Cable Tester.