Unfortunately the best .net decompiler will not be free anymore. The version 7 of .NET Reflector will not be free of charge anymore. For more details see red-gates announcement. The good news is that others took this as opportunity.
The next version of Jetbrains Resharper is bundled with a decompiler and a free standalone tool will follow.
Another tool comes from the SharpDevelop Community. For more information about ILSpy see Daniel Grunwalds blog.