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.
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.