Microsoft Research Detours - Detours is a software package for re-routing Win32 APIs underneath applications.
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
HookInjEx - How to inject code into another processes address space, and then execute it in the context of this process.
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
EasyHook - The reinvention of Windows API Hooking
madCodeHook - "madCodeHook" offers everything you need to hook code (mostly APIs).