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

ILSpy Features:

Assembly browsing
IL Disassembly
Support C# 5.0 “async”
Decompilation to C#
Supports lambdas and ‘yield return’
Shows XML documentation
Decompilation to VB
Saving of resources
Save decompiled assembly as .csproj
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)
Assembly Lists