Apigee - Intelligent and complete API platform
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
Apiary - Collaborative design, instant API mock, generated documentation, integrated code samples, debugging and automated testing
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
Postman - The Collaboration Platform for API Development
EasyHook - The reinvention of Windows API Hooking