.NET Reflector is an excellent assembly inspector for .NET, however it has became a commercial product under RedGate.
Fortunately, there is a free tools named ILSpy. It is developed by #Develop team. The GUI is done with WPF and uses some #Develop’s components, and it uses Mono.Cecil to do reflection.
FYI, Jetbrains will also release a free .NET Decompiler named DotPeek, but I haven’t tried it. It is in early access stage now.