Reflexil 
The .NET Assembly Editor
Reflexil is an assembly editor and runs as a plug-in for Red Gate's Reflector, ILSpy and Telerik's JustDecompile. Reflexil is using Mono.Cecil, written by Jb Evain and is able to manipulate IL code and save the modified assemblies to disk. Reflexil also supports C#/VB.NET code injection.
Homepage: http://reflexil.net
Howto: http://www.codeproject.com/KB/msil/reflexil.aspx
Compatible with:
- ILSpy
- Reflector
- Telerik JustDecompile
Videos:
- Converting a .NET GUI application to console using Reflexil and ILSpy (http://bit.ly/1H5RDdh)
- Unity3D assembly patching (AngryBots game) with Reflexil (http://bit.ly/un1ty)
- Playing with Reflexil and Reflector (http://bit.ly/kill3rv1d)
Download stable releases here: https://github.com/sailro/Reflexil/releases
or nightly releases here: https://sailro.visualstudio.com/Reflexil/_build?definitionId=2&_a=summary&view=runs