Apigee - Intelligent and complete API platform
WinAPIOverride - WinAPIOverride : This software allows you to monitor and/or modify any function of a process for any calling convention (stdcall or cdecl)
Apiary - Collaborative design, instant API mock, generated documentation, integrated code samples, debugging and automated testing
Microsoft Research Detours - Detours is a software package for re-routing Win32 APIs underneath applications.
Postman - The Collaboration Platform for API Development
HookInjEx - How to inject code into another processes address space, and then execute it in the context of this process.