In this post I will compile a list of tools that I use. Feel free to suggest any other tool :
Common Tools
WinDirStat reads the whole directory tree once and then presents it in three useful views.
http://windirstat.info/
Dependency Walker is a free utility that scans any 32-bit or 64-bit Windows module (exe, dll, ocx, sys, etc.) and builds a hierarchical tree diagram of all dependent modules.
http://www.dependencywalker.com/
Process Explorer shows you information about which handles and DLLs processes have opened or loaded.
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Managed
Deliver The Power Of Spy++ To Windows Forms
http://msdn.microsoft.com/en-us/magazine/cc163617.aspx
Explore, browse, and analyze .NET assemblies
http://www.red-gate.com/products/reflector/
Native
Spy++ (Available with Visual Studio.)
Profiling
AQTime ($)
http://www.automatedqa.com/products/aqtime/
Visual Studio Addin
Instantly find every occurrence of a function, variable, API, error code or anything else in your source code, with a single keystroke.
http://entrian.com/source-search/
54fb17c3-a5bf-4364-ad93-042712d76607|0|.0