ILSpy is a free .Net assembly browser and decompiler. I came across this application recently while looking for a replacement for .Net Reflector.
This tool was actually built by developer community after Red Gate started charging for the .Net Reflector.
The interface is nice and clean, and has a look and feel similar to that of Visual Studio. It can even look at assemblies that are in the GAC (File -> Open From GAC).
Once you have opened an assembly, all methods, properties, etc will be shown in the browser window, and you can view source code by clicking on relevant item.
It also has a class analyzer, which shows dependencies (Instantiated By, Exposed By and Extension Methods) if there are any.
ILSpy is very quick and easy to use, and in my experience worked perfectly.
I had a custom dll and no source for it, but with this tool I managed to extract all source code, copy it into new project and build without problems.
Highly recommended for .Net developers.