I recommend Visual Studio Code and one of the following two extensions:

• Thunder Client
• REST Client

Either one isn't really the full picture - you'll ned to combine it with other extensions - such as a good JSON language extension (which will give you syntax highlighting, error checking, code folding/etc.

The most important extension is CoPilot. That's the killer feature which makes Visual Studio Code vastly better than Postman.

Thunder is very similar to Postman. Not much to say other than it works well, it's free, millions of people use it.

It's not really my cup of tea, but I do think it's better than Postman because you can use your own version control servers to collaborate with colleagues, which is generally better (and cheaper) than Postman's collaboration service in my opinion (you get diffs, code review, pull requests, history, etc etc for all your most important API tests).

Personally I prefer REST Client (also free, and has even more users than Thunder).

REST Client is really simple. It adds a new "HTTP" text file type. You simply type a HTTP request into the file and hit a hotkey (or click a button) to execute the request. And it shows you the response. Easy.

HTTP requests and responses are just plain text, and you can simply save those as files in your project. REST Client also has basic support for variables, API credentials, etc. Not quite as user friendly as Postman or Thunder Client, but it makes up for that by being straightforward and flexible.

CoPilot Chat, works with both, but having everything in plain text gives it more control over REST Client than Thunder Clinet you can write (and edit) your requests with a series of simple plain english prompts. E.g. "JSON request with a blog post body" will give you:

POST https://example.com/blog/posts
Content-Type: application/json

{
    "title": "My First Blog Post",
    "body": "This is the content of my first blog post. It's not very long, but it's a start!",
    "author": "John Doe",
    "tags": ["blogging", "first post"]
}

You might follow that up with "Add a UUID" or "Add a JWT auth header".

Copilot can answer questions too - e.g. "How do I unsubscribe a user with the Mailchimp API?" They use the "HTTP PATCH" request type - WTF.