For me, it’s an electric toothbrush. It doesn’t matter if you go with Sonicare or Oral-B, once you start using an electric toothbrush, regular toothbrushes don’t ever feel like they clean your teeth properly. The smooth plaque-free top layer of your teeth that you can feel after using an electric toothbrush can’t be replicated with a regular toothbrush.
LAN tester.
I thought of it as fancy electrician / network equipment. Not anymore. Now it’s basic troubleshooting / procedure.
On a particularly frustrating switch installation, I picked one up for like $20 on Amazon, and it’s made me much less annoyed by network changes.
For context, I’m one of those people who hoards any electronic bits that might prove useful on a hobby project later, so lots of old patch cables and cable reels with unknown breaks, so maybe a LAN tester is really only worth it for others like that, but I’d recommend it to any level of tech enthusiast at least.