In no particular order. Year is when the last major work on it happened and more for my own overview than supposed to be really useful.
There are also some tools on my GitHub: github.com/lgommans

Diagnostic tools
2019 ip/ Reports $_SERVER[REMOTE_ADDR]
2020 inet/ Connection stability tests
2018 slomotest.htm Object moving at a fixed speed to measure slow-motion or frame rate

Services
2021 dro.pm link shortener, pastebin, file uploader in one. Shortest links ever.
2016 presentation-feedback/ Presentation feedback tool
2016 series-counter/ Tiny tool to keep track of where you were
2018 cau/ Content-addressed URL
2017 topstack/ An endless feed of interesting questions from StackExchange

Calculators
2019 pwd-combinations.html Calculate entropy given password requirements
2017 pwd-scrambler.php Allow a trusted person that should not know the password to log in
2020 tiny/g-force-distance-calculator.php g-force calculator (made for use on r/TheExpanse)

Web security
2020 cors-checker.htm Checks if it can fetch stuff from another page
2020 csp-test.php Allows you to set a content-security policy header for testing purposes

Miscellaneous
2017 rnd/whenimg.php Countdown image to be included in another website
2019 tiny/polar-tcx-garmin-converter.php Fix TCX files to make them portable from Polar to Garmin
2020 hangman-solver Solves hangman games
2021 badgravity Fly a spacecraft in orbit
2021 typing-speed See which keystrokes or words take you the longest