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 an application allowing you to display the current time in many time zones in the system tray.
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.)