.NET Reflector v6 –Free
Explore and analyze compiled .NET assemblies, viewing them in C#, Visual Basic, and IL.
New features:
+ Jump straight to .NET Reflector from Visual Studio
+ Support for .NET 4 assemblies
.NET Reflector Pro
.NET Reflector Pro is an add-in to Visual Studio that lets you debug third-party code and assemblies, even if you don't have the source.
+ Integrates the power of .NET Reflector into Visual Studio
+ Decompile third-party assemblies from within VS
+ Step through decompiled assemblies and use all the debugging techniques you would use on your own code