Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready for you to run on your own servers.

With API.AI, you can create conversational scenarios within minutes, then build advanced dialogues to manage the conversation flow with the user. Once you build your agent, you can integrate it with SDKs or use our one-click integration modules. When your product is launched, it constantly improves with machine learning and can be updated in real-time based on user interactions.

MailHog is an email testing tool for developers:

Configure your application to use MailHog for SMTP delivery
View messages in the web UI, or retrieve them with the JSON API
Optionally release messages to real SMTP servers for delivery

Mockbin allows you to generate custom endpoints to test, mock, and track HTTP requests & responses between libraries, sockets and APIs.

Swagger is a powerful open source framework backed by a large ecosystem of tools that helps you design, build, document, and consume your RESTful APIs.

Webhook Tester allows you to easily test webhooks and other types of HTTP requests.

GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. GraphQL provides a complete and understandable description of the data in your API, gives clients the power to ask for exactly what they need and nothing more, makes it easier to evolve APIs over time, and enables powerful developer tools.

Natural Language Interface for apps and devices. Build bots easily. You tell us what your user said, we tell you what your bot should do next. Your users give us voice or text, you get back structured data. It's that simple.

RethinkDB Horizon is an open-source developer platform for building realtime, scalable web apps. It is built on top of RethinkDB, and allows app developers to get started with building modern, engaging apps without writing any backend code.

Horizon consists of three components:

Horizon server: a middleware server that connects to/is built on top of RethinkDB, and exposes a simple API/protocol to front-end applications.
Horizon client: a JavaScript client library that wraps Horizon server’s protocol in a convenient API for front-end developers.
Horizon CLI: a command line tool, hz, aiding in scaffolding, development, and deployment.

Frisby is a REST API testing framework built on node.js and Jasmine that makes
testing API endpoints easy, fast, and fun. Read below for a quick overview, or
check out the API documentation.

Save requests and organize your APIs into Collections. Create and send any HTTP request using the awesome Postman request builder. Write your own test cases to validate response data, response times, and more! Collaborate in real-time over your team’s shared APIs. Make sure everyone is always on the same page. Test public APIs in one click using the Run in Postman button.

DuckRails is a development tool.

Its main purpose is to allow developers to quickly mock API endpoints that for many possible reasons can't reach at a specific time.

Matrix is an open standard for decentralised communication, providing simple HTTP APIs and open source reference implementations for securely distributing and persisting JSON over an open federation of servers.

Want to use Vanilla JS but find native APIs a bit unwieldy? Bliss is for you

Secure, Manage & Extend your APIs and Microservices

The open-source management layer for APIs, delivering high performance and reliability.

• RESTful Interface
• Plugin Oriented
• Platform Agnostic

htty is a console application for interacting with web servers. It’s a fun way to explore web APIs and to learn the ins and outs of HTTP.

A collection of the best open data sets and open-source tools for data science, wrapped in an easy-to-use REST/JSON API with command line, Python and Javascript interfaces. Available as a self-contained Vagrant VM or EC2 AMI that you can deploy yourself.

Cliquet is a toolkit to ease the implementation of HTTP microservices, such as data-driven REST APIs.

Fast, JavaScript DSP library that creates JIT optimized audio callbacks using code generation techniques.
It is designed to be a fast audio API for browser based audio content.

A specification for building APIs in JSON. Clients built around JSON API are able to take advantage of its features around efficiently caching responses, sometimes eliminating network requests entirely.