Free
JSON · Data Formats
The classic JSON validator and formatter.
jsonvalidatorformatter
Browse the complete directory — use filters to narrow down.
JSON · Data Formats
The classic JSON validator and formatter.
API Mocking · API Tools
Free fake REST API for prototyping and testing.
Notebooks & Data Science · Data & Analytics
The standard interactive notebooks for data science and ML.
JWT · Encoding & Decoding
The official JSON Web Token site: decode, debug and find libraries.
Web Servers & Proxies · DevOps
Popular open-source API gateway and microservices management.
Agent & LLM Frameworks · AI for Developers
Open-source LLM observability: traces, evals and prompt management.
Inspiration & Galleries · Design & Prototyping
Jon Yablonski's collection of UX maxims (Hick's Law, Fitts's Law…) with citations and practical takeaways.
Generators · CSS Tools
Interactive CSS Grid layout builder with code export.
Cheatsheets & Learning · Git & Version Control
Interactive visualization game for learning Git branching.
SSL & TLS · Security
Diagnose why Let's Encrypt certificate issuance fails for a domain.
Models & LLM APIs · AI for Developers
Open-source, multi-provider chat UI with agents and artifacts.
Models & LLM APIs · AI for Developers
Open-source LLM gateway: one API for 100+ providers.
Models & LLM APIs · AI for Developers
Meta's open-weight LLM family with downloadable weights.
Models & LLM APIs · AI for Developers
Open-source AI chat framework with plugins and agent marketplace.
Authentication · API Tools
Open-source auth infrastructure with a beautiful admin console — OIDC/OAuth, SSO and user management, self-hostable.
Logs & Metrics · Monitoring & Observability
Horizontally-scalable log aggregation designed like Prometheus.
Email · API Tools
Self-hosted email testing tool with a web UI, catches local mail.
Relational · Database
Community-developed MySQL fork with a permissive license.
Markdown · PDF & Documents
Free reference and tutorial for Markdown syntax.
Design Systems · Design & Prototyping
Google's open-source design system with guidelines and tokens.
MCP · AI for Developers
The open standard connecting AI models to tools and data sources.
References · Documentation & Learning
The definitive reference for HTML, CSS and JavaScript, by Mozilla.
Search Engines · Database
Fast, typo-tolerant search engine with great defaults (trending).
NoSQL · Database
The classic, simple distributed memory object cache.