Cable ties (reuseable)
Gaffa Tape (Green and Black) - The green is for my stuff, the black for other peoples...
Double sided tape
Small tool kit with pliers, adjustable spanner, tweezers, screwdrivers, nippers, allen keys)
2 x 9V batteries
Spare "watch" batteries
Fuses
Gas soldering iron and solder
crimp connector kit
1 x 6 way surge protected extension leads
cable tester
multimeter
spare instrument, patch and speaker cables. Spare connectors (Speakon, 6.35mm, Phono, XLR)
various connector/adaptors/gender changers/extenders
kettle lead
tuner
towel
spare bass strap
water
spare strings (never used)
spare 9V power supply with multi end adaptor.
With the exception of the strings I've needed all of these or wanted to have them at a gig/rehearsal at some point... I use a soft camera bag (like a small-medium sized narrow backpack) which keeps things vaguely organised...However it's too small for my guitar stand, which is no biggy as I have lots of other stuff to lug as well...
cheers
Mark