Tools

Mindmap, Whiteboards & Diagrams

Excalidraw - sketching hand-drawn diagrams in the browser (no accoount needed)
Draw.io - webtool for creating all sorts of diagrams
DBDiagram.io - Webtool to create DB Diagrams

Markdown

TableGenerator - Generating markdown Tables. Also for LaTex, HTML...

Others

Open Alternative - List of open source alternatives for popular software

Data

JSONCrack - JSON / XML Tree visualiser -> VS Code Plugin
JSON Formatter - Chrome plugin to format raw JSON in the browser

Desktop

Greenshot - Screenshot Tool on Steroids (it can do annotations and more)

Development

Jam.dev - Share Bugs in Webapps with replay

Research

Connected Papers - Graph view for related papers
Inciteful - Find connected papers and search how papers relate
Research Rabbit - Academic Research tool (free account required)
Epsilon.ai - AI Search tool to find relevant research to your question
Storm - E2E AI research tool from stanford