Here are a few projects that I've worked on in my spare time. So far these are all of the "fits entirely in a serverless single-html file" variety.
A Java library representing unlimited precision fractions. Available via Maven Central.
This is an application allowing you to pick the notes in a guitar chord, and it shows the possible names for that chord, and also allows you to hear what the chord will sound like.
This is a simple app that figures out the exact date that was half a lifetime ago for you.
A very lightweight app to prettify/uglify JSON data directly in the browser.
A powerful in-browser regular expression evaluator, hidden behind an opaque developer UI. (I wrote this for me so I don't have any plans to make the UI more user-friendly. Sorry not sorry.)