It’s convenient that the 5.5/2.1 mm center-positive DC connector is nearly ubiquitous among DIY electronics. On the other hand, the “whoops I grabbed the wrong supply and killed my LED strip” tragedy is almost a weekly occurrence in the forums. I’ve done it too. They all look the same.
You don’t need a fancy labeler for this (but they’re fun!). Tape will do. Or bread clips. Whatever, avoid the Magic Blue Smoke Monster and label all your power supplies one way or another.
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Sharpie Markers – the metallic “color”. Or “Paint Pens”.
When I get a new supply, I write what it went to (XYZ phone, RasPi, etc) on the “box” part of the supply and/or the voltage – often on the plug itself, or something to identify it
Second vote for metallic Sharpies. I write the voltage and current outputs on the wall wart portion of the supply.
Also handy to indicate the rated current to avoid using a 350mA wart for a device that will draw 1A.
What TZ said about including the data on which device the power supply originally belongs with. Since so many of the supplies are labeled with the name of some OEM power supply manufacturer, rather than anything that matches what it attaches to!
Maybe a polarity note at the wire end as well? One side could say “12V,” and the other “tip +” or whatever your wallwart is.