1) Process Moniter (procmon)
If you need to moniter whats happening with your file system or registry, just open this tool and this will record all events at OS level. You can filter them according to your needs. I used this to debug one of installer issue where the installer is supposed to read from registry but was reading from wrong location because it was a 64bit OS machine.
2) Fuslogvw (Fusion log viewer)
Use this tool to check assembly bindings at runtime.
3) Debugview (DbgView.exe)
Use this tool to debug traces that you can write in your .NET applications using System.Diagnostics.Trace.Write(“IIS site activated”). This will helpful in debugging application in UAT and PROD environment.