A (growing) list of utilities I use frequently:

Regular Expressions

https://regexr.com/ – visual regular expression builder and tester

Logging

http://log4jtester.com/ – visual tester for log4j conversion patterns

Dates and Times

http://www.sdfonlinetester.info/ – SimpleDateFormatter online tester

https://www.epochconverter.com/ – Epock and Unix timestamp conversion tools

http://www.cronmaker.com/ – Cron job expression generator

Encoding / Decoding

https://meyerweb.com/eric/tools/dencoder/ – URL decoder and encoder

https://bcrypt-generator.com/ – Encrypt and decrypt tester

Electronics

https://www.allaboutcircuits.com/tools/resistor-color-code-calculator/ – resistor color codes

Graphics

http://www.imageoptimizer.net/Pages/Home.aspx – image optimizer (online image resize and compression)

CSS and HTML

https://www.rapidtables.com/web/css/css-color.html – CSS colors, sorted by rainbow instead of name

Miscellaneous

https://www.lipsum.com/ – test text generator

https://qph.fs.quoracdn.net/main-qimg-d438660397b6510841781c97ee74578e – complexity of data structure elements and sorting algorithms