According to RedGate .NET Reflector is longer a free (as of February 2011, which leaves a big opening for a new assembly browser for .NET assemblies.
Today I checked out ILSpy. It is an open-source .NET assembly browser and decompiler. It runs on .NET 4.0, and from GitHub is appears to be maintained. It even has a couple of plug-ins (I didn't install any of them) available: http://wiki.sharpdevelop.net/ILSpy%20Plugins.ashx
The user interface looks familiar, almost like .NET Reflector from a few versions ago. It also supports the drag-and-drop of assemblies.
It's simple and works well. I'm going to keep it.
Next up I am going to install the Visual Studio Plug-In and see
ILSpy in GitHub
ILSpy Visual Studio Plug-In (