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
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).
EasyHook - The reinvention of Windows API Hooking
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
madCodeHook - "madCodeHook" offers everything you need to hook code (mostly APIs).
Mhook - Mhook is a library for installing API hooks.