Stuff I make

Countdowny bits

Atropine - The browser-based tool used by most FOCAL Countdown events to generate fixtures and record results. It is also available on GitHub.
Mobile numbers solver - Countdown numbers game solver, written in JavaScript and designed for use on mobile devices.
Quantum Tombola - same numbers solver, different interface. It has more features and is designed for devices with larger screens and an actual keyboard.
WebRack - the system ZoomDown uses to run games over Zoom and show overlay graphics.
RackONine - letters and numbers picker that some have found useful in online events.
Conundrum Printing Thing - Lay out conundrum scrambles in a printable form, for a Lincoln-style co-event.
Scoresheet printer - Like the Conundrum Printing Thing, but for co-event scoresheets.
H2H-o-matic - an occasionally-updated set of pages showing statistics about past co-events.
The nets - Numbers game trainer.
Productio ad absurdum - Trainer for some useful times tables.
Drumcundry - A thing I barely remember making in 2016, with which you can practice against real conundrums that have been used on Countdown.

Octagon Simulator - A simulation of the octagonal on-screen countdown clock used by early 90s BBC shows and referred to here.

Bitlump - a tool to convert numbers to other bases and various integer sizes, and to break down strings into their Unicode codepoints.

ton - a command-line application for Linux and Windows to make transferring a file from one computer to another easier than might be otherwise.

Binomial probability calculator - a JavaScript tool to work out binomial probabilities without all the cruft and popups which adorn some other such calculators.
The Knight Errant puzzle - a problem about knight's tours and semimagic squares which I found in an old puzzle book, and a program to find additional solutions.

Star Viewer, or "Watson, Someone Has Stolen Our Tent" - in 2010 I made this thing that generates the position of all the stars and planets in the sky for a given location. There are probably better solutions to this problem now.