List of free api-testing stuff for developerππ½
If you're looking for all programming tools around api-testing (not just free). Check this page List of api-testing Programming Tools

HTTPie
Making APIs simple and intuitive for those building the tools of our time.
Tag: API, API-testing, API-client

Open Source Free updates forever API Client Design APIs Plugins Community Support (Github and Slack)

postman
Postman makes API development easy. Our platform offers the tools to simplify each step of the API building process and streamlines collaboration so you can create better APIs faster.
Tag: API, API-testing, API-client
Unlimited shared requests for up to 3 team members

Thunder Client
Thunder Client is a Http Client Extension for Visual Studio Code for Testing APIs. Supports Graphql query. Rest Client lightweight alternative to Postman
Tag: API, API-Testing, API-Client
Lightweight API Client Simple, Clean & Easy to use UI Handle Large Responses & View in Full Screen Support VS Code Themes Collections & Environment Variables Scriptless Testing

Hoppscotch
Helps you create requests faster, saving precious time on development.
Tag: API, API-testing
Open Source

Firecamp
Firecamp is a multi-protocol API development platform. It offers dedicated GUI clients, tools and services for legacy to modern protocols and styles like Http/Rest, GraphQL, WebSocket, MQTT etc.
Tag: api-testing, api
Collaboration within your team Automate Cross API testing Protocol buffer testing Multiple API specifications Centralized platform for your API development