+1 for Furman AR-1215. I carry this unit in a separate 2-space rack (it's a 1U device) to every gig. When we play in larger venues, there are dedicated 20A circuits available for stage power. But in smaller clubs this device is crucial to manage fluctuating supply voltage. Sometimes the stage power is fed from the same circuit as the lights, or the kitchen deep-fryer or something. Many of us are using Class D amps, which are fantastic for their light weight; they can be very sensitive to fluctuating line voltage, and the AR-1215 really stabilizes those amps.
By the way, I'd strongly recommend purchasing a couple of those $1.99 plug-in circuit testers from your local hardware store. When my band gets ready to set up onstage, the first thing I do is take a quick sweep of the stage power supply by plugging a circuit tester into each stage outlet. I don't know how many times I've discovered (a) an open ground (b) a hot-neutral reversal or (c) a dead outlet. I identify each outlet with a piece of board tape to indicate my findings, marking good outlets and taping over bad ones. It sure saves time in gremlin-hunting. I also carry the tools necessary to deal with those basic issues; when it's necessary to address a wiring problem I'll discuss the situation with the management, and with their blessing, shut down breakers to correct outlet wiring issues before we set up.
Scarier still: Once we arrived at a club where we had played many times. I did my usual outlet test and when I plugged my tester into the stage power where I usually set up, the tester immediately went FZZZZZT and emitted a tiny sad mushroom cloud. I checked the outlet with the multimeter; the club management had thoughtfully pulled new circuits to the stage since our last show there, but that whole bank of outlets was wired for 220v rather than 110v. The installer had wired both hot feeds to the outlet rather than splitting them for two 110v sides (!). It was a simple fix, and once corrected, we had really great stable power. My $1.99 tester went to heaven, but it could have been much worse. I'd like to think fuse-protected gear would be safe... but I'd have been mightily grumpy if my Boogie's xformer were blown.
And finally: I have training and experience dealing with electrical wiring. Don't mess with it unless you do too. I always notify the venue if I find something peculiar. I offer to address it only if it is necessary for our show to proceed; otherwise I just indicate the outlet(s) that are marked with board tape, and let them handle it.