ILSpy 

Access .NET assembly files and analyze the construction, integrity and specific components of products. The program provides virtual disassembly option down to decompilation to the C# code, supports lambdas and 'yield return', etc. Export to and import from XML documents is possible.

ILSpy is the open-source .NET assembly browser and decompiler.

Main features:

- Assembly browsing

- IL Disassembly

- Decompilation to C#

- Supports lambdas and 'yield return'

- Shows XML documentation

- Saving of resources

- Search for types/methods/properties (substring)

- Hyperlink-based type/method/property navigation

- Base/Derived types navigation

- Navigation history

- BAML to XAML decompiler

- Save Assembly as C# Project

- Find usage of field/method

- Extensible via plugins (MEF)