Flash news!
JetBrains released the 1.0 (RTM) version of their FREE decompiler - dotPeek. It’s a viable options for all those who can’t live without RedGate Reflector but don’t want to pay for it. You can find it at http://www.jetbrains.com/decompiler/
Of course dotPeek is not an only free docompiler available - Telerik fans can get JustDecompile (http://www.telerik.com/products/decompiler.aspx).