ILSpy

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)