My side projects tend to be small and just about fill a hole in the software space that has gone unfilled.
Sometimes they're very quickly written and not a great example of amazing code.
Productivity
FlowRatio: The best time tracking tool you'll ever use. This is actually as unobtrusive as other tools claim to be, but still very accurate as long as one is honest when it prompts.
Decision Drill: Collaborative brainstorming and dot voting made anonymous and simple.
Engineering Enigmas: Over 150 worthwhile dilemmas for any creative block.
Tagnostic: Tags are difficult. Have a conversation with your tag assignments and see if they're any good.
Swemak: Are Swedish, but want to use Colemak? This is one of the best solutions.
Games
Flightle: A sidescrolling flight simulator with somewhat proper flight dynamics.
Ship Investor series: Practice your binary Kelly criterion in the first ship investor, or continuous allocation in the second. Play the first Ship Investor, or try your hand at the second Ship Investor.
logtrain: Three scripts that together get you to focus on practicing the logarithms you're not very good at. Become a mental mathemagician!
Statistics and Forecasting
xmr.html: Make scrappy control charts in no time, from anywhere. This is the most powerful statistical tool even a layman can get their hands on.
Kelly Insurance Calculator: Use real maths to figure out whether your insurance is really worth it. Don't base the decision on vibes.
Fisher's Fountain: Ever needed a table of random Poisson draws, or something else? Despair no more.
Wactomo: Interactive tool for comparative statics in actor–motivation forecasting à la Bueno de Mesquita.
Precel: A postfix syntax and evaluator for arithmetic on uncertain values using resampling.