Web:
https://fonts.google.com/
https://closure-compiler.appspot.com/home
https://www.iconfinder.com/
https://loading.io/
http://bg.siteorigin.com/
https://csscompressor.com/
https://www.colorzilla.com/gradient-editor/
https://enjoycss.com/
https://javascript-compressor.com/
https://favicon.io/

Programmering:
https://regex101.com/

Diverse:
https://www.diffchecker.com/
http://www.falstad.com/circuit/circuitjs.html
https://www.hobby-hour.com/electronics/ledcalc.php
https://www.portablefreeware.com/