If you haven't read Daniel's post already - ILSpy is open-source alternative to the .NET Reflector licensed under MIT/X11.
I have uploaded my decompiler engine to the github repository and Daniel has already integrated it with the ILSpy GUI. The decompiler is based on my university dissertation which you can read on github if you are interested. This was actually a piece of code that was just lying on my harddrive for two years. Since .NET Reflector will no longer be free, I finally have the proper motivation to finish it. See below for a screenshot of the first version.
I am currently in the process of cleaning up the code to make sure that it will be maintainable in the long term and that other people can more easily contribute. One of the main goals of the decompiler will be to ensure correctness of the decompiled program. The decompiled code should be accurate representation of IL bytecode. I will write several more posts about the decompiler in the following days.