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.
Structurizr
Implements the C4 model as a diagrams-as-code DSL — write Structurizr DSL and render multiple architecture views (Context, Container, Component, Code) from a single model. Version-controlled alongside code with full ADR integration. Free OSS CLI plus paid cloud workspace for team collaboration.