There are a lot of things that should be in your toolbox/utility suitcase/rack drawer/etc at a gig, and most of these are common knowledge. Spare cables are a big one: not just 1/4″ but a spare power cable, maybe a spare midi/footswitch cable if applicable. Also, an extension cord, spare strings, a string cutter, a battery powered tuner (even if you have one on your pedalboard or rack), a screwdriver, spare patch cables, velcro, etc.
But there are two often overlooked items that shouldn’t be: spare tubes (assuming you play through a tube amp) and fuses. Now I have gigged without spare tubes. They can be expensive, and you need a pair (usually), and if you’re not on tour it may be easier to borrow a head from another band instead of swapping out tubes in the middle of your 30 minute set. And did I mention they can be expensive?
But fuses? Seriously, every guitarist and bassist should have a spare set. Yet it’s amazing how rarely people carry them. Sometimes your amp is perfectly fine but just plugged into an outlet with shoddy power and your fuse blows. You’re going to need a spare eventually. But more to my point, a 4-pack of fuses will run you something like $2-$4 at Radio Shack. So just pick some up. Maybe you won’t need them for years. But one day your fuse will blow and you’ll feel like a dumbass for not having a backup.
And by the way, I recommend keeping them in a pill holder or something. Otherwise once you rip open that bag to get at one fuse you’re guaranteed to lose the other 3. I have an AM/PM pill holder that holds my fuses in one side (“AM” is for “Amp”) and I keep a few spares for my forgetful bandmate in the other.