Spec Kit
Open-source scaffold from GitHub that operationalizes spec-driven development with AI coding agents. Creates structured .specify/ folders that serve as the source of truth for Claude Code, GitHub Copilot, and Gemini CLI — steering AI generation from upfront specs rather than improvised prompts.
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.