Some stuff I use often:

- Finding out what a linux command line tool's flag does without having to CTRL+F through multiple matches in the manfile (https://explainshell.com/)

- Test and visualize JS regex (https://regexr.com/)

- Render PlantUML class or sequence diagrams (https://www.planttext.com/)

- Unicode code converter (https://r12a.github.io/app-conversion/)

- Unicode codepoint viewer (https://r12a.github.io/uniview/)