Reflector for .NET
Reflector is the class browser, explorer, analyzer and documentation viewer for .NET. Reflector allows to easily view, navigate, search, decompile and analyze .NET assemblies in C#, Visual Basic, Delphi and IL.
download .NET Reflector

.NET Reflector is a very useful tool for .NET developer, and it supports Add-Ins, from here you can find many Add-Ins.