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.
Freemake Video Downloader(Windows): A program for downloading videos from YouTube so they can be watched when not connected to the internet. Unlike some others, this one works!
Get i-Player (Windows and Linux): A program which downloads anything available on the BBC i-Player. This is very handy if you have a dodgy or slow internet connection or wish to watch or listen to things when not connected. It was originally written for Linux so is about as user-friendly as a barbed-wire bog-roll but is great if you can work it out.
For those not scared of command lines a good Linux YouTube downloader is cclive. There's also youtube-dl which is available for both Windows and Linux and will also work with the BBC i-Player.
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.