SpyStudio - SpyStudio is the best product to trace user-mode API calls on Windows.
EasyHook - The reinvention of Windows API Hooking
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
Microsoft Research Detours - Detours is a software package for re-routing Win32 APIs underneath applications.
Process Monitor - Monitor file system, Registry, process, thread and DLL activity in real-time.
madCodeHook - "madCodeHook" offers everything you need to hook code (mostly APIs).