It is disheartening to arriving for a gig to find out that your guitar is not playing well, and not having any tools to try to make things right. At the least, a guitarist should make sure that they have a few basic tools with them at all time, and there are dozens of different multi-tools out there that fit the bill. I found my favorite a few years ago when I was in Japan and I needed to do some work on an axe I picked up over there. This tool is the Ibanez MTZ11.
The MTZ11 comes in cool colors (I chose red), and it is small enough to fit into any gig bag pocket. It measures around 3.75 inches long by 1.4 inches wide by 1.3 inches tall. It has 11 tools in it, including two Phillips head and a flat head screwdriver, 5 hex wrenches (1.5 to 5mm), a 7mm hex that will accept standard bits, and a 50mm ruler. These tools are made of high-grade steel, and I have not noted any wear to the screwdrivers or hex wrenches despite fairly heavy use.
But what really sets this one apart from the other tools on the market is the 50mm steel ruler. This is really handy for setting action and pick-up heights, and I would not buy a multi-tool that does not have a ruler in it if I had to buy another one.
The Ibanez MTZ11 multi-tool is cheaper than buying all of these tools separately, and it certainly is a tidy package. It is not terribly expensive, coming in at around $21 on Amazon, which is really cheap insurance if your gig is at stake. Check one out if you get the chance!