Mermaid
Renders flowcharts, sequence diagrams, ER diagrams, C4 diagrams, and Gantt charts from simple Markdown-style text definitions. Native support in GitHub, GitLab, Notion, and dozens of engineering platforms. With 87k+ GitHub stars, it's the most widely adopted diagramming tool in engineering.
Eraser
Generate architecture diagrams, ERDs, cloud infrastructure diagrams, and sequence diagrams from natural language or code via DiagramGPT. Supports GitHub sync and a VS Code extension so diagrams stay connected to code. SOC II Type 2 compliant and never trains on user data.