Cordless screw driver. Bought the day after building a flat pack bed with a crappy screw.driver that just shredded my hand. Thought it was frivolous at the time, but I've used it so much since. It's light, small enough to fit in my pocket and good for 90% of DIY tasks.
Got a gun from sako the other day it's cute, it's small, fits right in my pocket (yeeeeeah right in my pocket)
Real answers tho:
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Air pump for car tires. I have notoriously bad luck with flats, leaks, etc. This thing has saved me more than a few times.
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KVM switch. I work from home, this lets me have my work computer and my personal computer connected to the same equipment, and I can toggle between them with either a physical switch or a keyboard hotkey
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Bit of a curve ball, but therapy. Life's tough, gotta have some outlets and advice sometimes.