Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
madCodeHook - "madCodeHook" offers everything you need to hook code (mostly APIs).
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Microsoft Research Detours - Detours is a software package for re-routing Win32 APIs underneath applications.
Better Uptime - We call you when your website goes down
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).