How do I decompile a .NET EXE into readable C# source code?

Reflector and its add-in FileDisassembler.

Reflector will allow to see the source code. FileDisassembler will allow you to convert it into a VS solution.

When Red Gate said there would no longer be a free version of .Net Reflector, I started using ILSpy and Telerik's JustDecompile. I have found ILSpy to decompile more accurately than JustDecompile (which is still in Beta). Red Gate has changed their decision and still have a free version of .Net Reflector, but now I like ILSpy.

From the ILSpy website (https://github.com/icsharpcode/ILSpy/):

ILSpy is the open-source .NET assembly browser and decompiler.

ILSpy Features

• Assembly browsing
• IL Disassembly
• Decompilation to C#
• Supports lambdas and 'yield return'
• Shows XML documentation
• Saving of resources
• Search for types/methods/properties (substring)
• Hyperlink-based type/method/property navigation
• Base/Derived types navigation
• Navigation history
• BAML to XAML decompiler
• Save Assembly as C# Project
• Find usage of field/method
• Extensible via plugins (MEF)

Update:

April 15, 2012, ILSpy 2.0 was released. New features compared with version 1.0:

• Assembly Lists
• Support for decompiling Expression trees
• Support for lifted operatores on nullables
• Decompile to Visual Basic
• Search for multiple strings separated by space (searching for "Assembly manager" in ILSpy.exe would find AssemblyListManager)
• Clicking on a local variable will highlight all other occurrences of that variable
• Ctrl+F can be used to search within the decompiled code view

Update:

• ILSpy 2.1 supports async/await decompilation

Reflector is no longer free in general, but they do offer it for free to open source developers: http://reflectorblog.red-gate.com/2013/07/open-source/

But a few companies like DevExtras and JetBrains have created free alternatives:

DevExtras CodeReflect

JetBrains DotPeek