Electronics Assistant (Windows): A program that does electronics-related calculations: resistors in parallel, LM317s &c.
Electrodroid (Android): Similar to Electronics Assistant but much more comprehensive. As well as a myriad of calculators it also includes common, and not-so-common, pin-out information plus much more. It does contain advertising but not to a large degree.
yt-dlp (Windows and Linux): A fantastic program for downloading videos from YouTube and many other sites, including the BBC i-Player/BBC Sounds.
ConEmu (Windows): A replacement for the built-in Windows Command Prompt. It is much more powerful, in fact it's probably too powerful for we mere mortals. It has the great advantage of Cut and Paste working properly, rather than having to faff about with Right-Clicking the Title Bar then, Edit, Cut/Paste/Copy. It also allows the text size to be upped so it uses the whole screen making it easier to read.