MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.

madCodeHook - "madCodeHook" offers everything you need to hook code (mostly APIs).

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.

Deviare API Hook - Deviare is a professional open source hooking engine for instrumenting arbitrary Win32 functions, COM objects, and functions which symbols are located in program databases (PDBs).