This is the BEST .NET decompiler tool in the market. It allows you to decompile, understand, and fix any .NET code, even if you don’t have the source. However, .NET Refector is no longer free from Feb, 2011. You can watch its introduction video from here:
The rest tools in the list are free. They are good .NET Reflector alternatives.
ILSpy is the open-source .NET assembly browser and decompiler. You can find its latest source code on GitHub.
dotPeek is a free-of-charge .NET decompiler from JetBrains, the makers of ReSharper and more developer productivity tools.