MockIt (open source)

It is the only complete API development environment, used by nearly five million developers and more than 100,000 companies worldwide.

Insomnia is a powerful REST API Client with cookie management, environment variables, code generation, and authentication for Mac, Window, and Linux.

It is probably the most popular HTTP client library for .NET. Featuring automatic serialization and deserialization, request and response type detection, variety of authentications and other useful features

It is a publicly available application programming interface that provides developers with programmatic access to a proprietary software application or web service.

Retrofit turns your HTTP API into a Java interface

It defines a standard, language-agnostic interface to RESTful APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection.

API management, design, analytics, and security are at the heart of modern digital architecture. The Apigee intelligent API platform is a complete solution for moving business to the digital world.

It is an open source functional Testing tool for API Testing. It supports multiple protocols such as SOAP, REST, HTTP, JMS, AMF and JDBC. It supports functional tests, security tests, and virtualization.

Paw is a full-featured and beautifully designed Mac app that makes interaction with REST services delightful. Either you are an API maker or consumer, Paw helps you build HTTP requests, inspect the server's response and even generate client code.

It is a Swift-based HTTP networking library for iOS and macOS. It provides an elegant interface on top of Apple's Foundation networking stack that simplifies a number of common networking tasks.

It is an open source project which allows generation of API client libraries (SDK generation), server stubs, and documentation automatically from an OpenAPI Specification.

Combines API test-automation, mocks and performance-testing into a single, unified framework. The BDD syntax popularized by Cucumber is language-neutral, and easy for even non-programmers. Besides powerful JSON & XML assertions, you can run tests in parallel for speed - which is critical for HTTP API testing.

RESTful API Modeling Language (RAML) makes it easy to manage the whole API lifecycle from design to sharing. It's concise - you only write what you need to define - and reusable. It is machine readable API design that is actually human friendly.

Charles is a web proxy (HTTP Proxy / HTTP Monitor) that runs on your own computer. Your web browser (or any other Internet application) is then configured to access the Internet through Charles, and Charles is then able to record and display for you all of the data that is sent and received.

Created with <3 for front-end developers who need a quick back-end for prototyping and mocking.

Keep tabs on all aspects of your API's performance with uptime monitoring, integration testing, logging and real-time monitoring.

Newman is a command-line collection runner for Postman. It allows you to effortlessly run and test a Postman collection directly from the command-line. It is built with extensibility in mind so that you can easily integrate it with your continuous integration servers and build systems.

The automatic type-safe REST library for .NET Core, Xamarin and .NET. Heavily inspired by Square's Retrofit library, It turns your REST API into a live interface.

It is a free web debugging proxy for any browser, system or platform. It helps you debug web applications by capturing network traffic between the Internet and test computers. The tool enables you to inspect incoming and outgoing data to monitor and modify requests and responses before the browser receives them.

Stop writing thousands of lines of specification code. Our intuitive visual editors significantly cut down on design time, and are spec agnostic. Generate OAI (Swagger) and RAML specification code on demand.

API Blueprint is simple and accessible to everybody involved in the API lifecycle. Its syntax is concise yet expressive. With API Blueprint you can quickly design and prototype APIs to be created or document and test already deployed mission-critical APIs.

It is a complete, modern, intuitive and scalable API platform that lets you create, securely expose, manage and monetize APIs across clouds so that you and your customers can power digital applications and spur innovation.

Appwrite's open-source platform lets you add Auth, DBs, Functions and Storage to your product and build any application at any scale, own your data, and use your preferred coding languages and tools.

It delivers the only complete open source middleware platform. With its revolutionary componentized design, it is also the only open source platform-as-a-service for private and public clouds available today. With it, seamless migration and integration between servers, private clouds, and public clouds is now a reality.

Generate real Swagger/OpenAPI 2.0 specifications from a Django Rest Framework API.

HTTP is the way modern applications network. It’s how we exchange data & media. Doing HTTP efficiently makes your stuff load faster and saves bandwidth.

It is an integrated API Development platform, built for teams, that brings the core capabilities of the Swagger framework to design, build, document and deploy APIs.

Legible & concise status badges for third-party codebase services.

It allows you to easily build & consume fully typesafe APIs without schemas or code generation. It is well-tested and production ready. Experience the full power of TypeScript inference to boost productivity for your full-stack application.

Its core functionality is to create an API that acts as an aggregator of many microservices into single endpoints, doing the heavy-lifting automatically for you: aggregate, transform, filter, decode, throttle, auth and more.

It is a code-first tool for testing HTTP and message integrations using contract tests. Contract tests assert that inter-application messages conform to a shared understanding that is documented in a contract. Without contract testing, the only way to ensure that applications will work correctly together is by using expensive and brittle integration tests.

It is a free, fast, and beautiful alternative to Postman. It helps you create your requests faster, saving you precious time on your development.

It is a free, fast and beautiful API request builder. It helps you create requests faster, saving precious time on development

An API-based architecture is essential to agile delivery of mobile applications. Apigility provides JSON representations that can be parsed and used in any mobile framework; write for the web or native applications simultaneously!

An NGINX-based proxy and distributed architecture give unparalleled performance and scalability. Using an Open API Specification or one of our API frameworks, Cloud Endpoints gives you the tools you need for every phase of API development and provides insight with Google Cloud Monitoring, Cloud Trace, Google Cloud Logging and Cloud Trace.

It is a flexible, lightweight and blazing-fast open source API Platform that helps your organization control finely who, when and how users access your APIs.

It is a free cloud-based API testing and documentation tool to simplify the validation of any API and generate its corresponding OpenAPI documentation.

Quickly mock RESTful API or SOAP web-services with simple or dynamic responses, and fault injection to simulate real application behaviour.

Falcor lets you represent all your remote data sources as a single domain model via a virtual JSON graph. You code the same way no matter where the data is, whether in memory on the client or over the network on the server.

It is a framework with integrated OpenAPI compiler to build Node.js serve-side applications using TypeScript. It can target express, hapi, koa and more frameworks at runtime. tsoa applications are type-safe by default and handle runtime validation seamlessly.

Checkly is the API & E2E monitoring platform for the modern stack: programmable, flexible and loving JavaScript. Free for developers!

It is an open-source bulk data loader that helps data transfer between various databases, storages, file formats, and cloud services.

Deployd is the simplest way to build realtime APIs for web and mobile apps. Ready-made, configurable Resources add common functionality to a Deployd backend, which can be further customized with JavaScript Events.

It is built from the ground up with the Local-first paradigm in mind. This allows developers to directly store their collections on top of their local filesystem. The collections are mirrored as folders and files on the filesystem.

Packaged as a Chrome extension and accessible from any modern web browser, Restlet Studio instantly provides to any developer powerful API crafting capabilities. Through an intuitive interface, developers can visually define all the elements of a web API, and automatically generate the client SDK and server skeleton of their API.

Thunder Client is a Rest API Client Extension for Visual Studio Code, with simple and clean design. The extension makes api testing very easy with Scriptless GUI based testing.

It lets you build cloud-native microservices API gateways, delivering the ultimate performance, security, open-source and scalable platform for all your APIs and microservices.

It is an API mocking library that lets you build, test and share a complete working JavaScript application without having to rely on any backend services.

IBM API Connect Test and Monitor provides automated, collaborative, and powerful API testing and monitoring, with no code needed.

It is a complete platform for building, integrating, and exposing your digital services as managed APIs in the cloud, on-premise, and hybrid architectures to drive your digital transformation strategy. It allows API developers to design, publish, and manage the lifecycle of APIs and API product managers to create API products from one or more APIs.