Diagramming Tools

MermaidJS
PlantUML (Uses PlantUML Server for rendering)
D2
Diagrams.net (SVG; More diagrams than PlantUML/Mermaid; Open-Source; Formerly Draw.io)
Excalidraw (Sketching)
Markmap (Mindmaps)
GraphViz (Generates directed graphs from text descriptions using Dot)