Process Monitor - Monitor file system, Registry, process, thread and DLL activity in real-time.
Microsoft Research Detours - Detours is a software package for re-routing Win32 APIs underneath applications.
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
EasyHook - The reinvention of Windows API Hooking
perf - Perf is a simple app monitoring solution paired with meaningful alerts.
madCodeHook - "madCodeHook" offers everything you need to hook code (mostly APIs).