There is a big difference between a free service and open source. For example GSuite is free for personal usage. But the software is not open source at all.
Most people don't care, but I do.
If possible I prefer open source solutions.
- A tiny little drawing app tldraw.com
- Make images smaller using best-in-class codecs, right in the browser: squoosh.app
- Virtual whiteboard for sketching hand-drawn like diagrams excalidraw
- http API Client (Web GUI): Hoppscotch
- LimeSurvey
- VTracer Convert PNG to SVG.
- mermaid (text based diagramming and charting tool) LiveEditor
- scrumlr.io Webapp for collaborative online retrospectives
- Offline Maps and Navigation Osmand