MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
API Monitor - API Monitor is a software that monitors and displays API calls made by applications and services. Its a powerful tool for seeing how Windows and other applications work or tracking down problems that you have in your own applications
Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
Microsoft Research Detours - Detours is a software package for re-routing Win32 APIs underneath applications.
Webhook.site - Instantly generate a free, unique URL and email address to test, inspect, and automate (with a visual workflow editor and scripts) incoming HTTP requests and emails.
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.