Friendly serial/tcp/telnet/ssh gui client - Windows and unix

It’s a disposable email platform made for QA and test automation. You get private, isolated inboxes that behave like real ones — but are built for fast, repeatable, testable flows.

Free & OpenSource REST API Testing Tool for all levels in a Test Pyramid

Postman is an API platform for building and using APIs. Postman simplifies each step of the API lifecycle and streamlines collaboration so you can create better APIs—faster.

Design, debug, and test APIs locally or in the cloud

Burp or Burp Suite is a set of tools used for penetration testing of web applications. It is developed by a company named Portswigger who make freely available a Web Security Academy which has a range of lessons on penetration testing.