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As others have said, it depends where you draw the line on what is a "gadget". But ignoring some of the obvious ones that have already been mentioned several times: I think I might have to go with my desktop audio mixing board. It lets me easily mix sound from multiple computers/devices and run them all into one set of headphones or speakers. I can adjust the volume of each input independently and without futzing with the settings in individual software applications/systems.

My other mention would go to my Raspberry Pi's. Incredibly compact, low-power-usage, but potent, versatile computers. Even just one or two can power a broad suite of at-home/self-hosted services, removing the need for costly, privacy-invasive/insecure, third-party services.