A must to have for every software developer are so called disassemblers. They will help you out, when you've lost the source code of a DLL or you need to analyze a bug, or verify the right version of an assembly. I had to update this article in 2011 to find a new free disassembler and was lucky to find a good alternative.
Since the 1st of March 2011, Lutz Roeder's .net Reflector (a .net disassembler) is not available for free anymore. I had to look for a free and good alternative and found one.
So far there were several different and open source projects, I now started to use the following program that I recommend. It's called ILSpy:
It works pretty much the same than other products and can convert an entire .net DLL into a C# project if needed. It's easy to use and pretty small, the version I've tested was 1.0.0.1000.
Be aware that sourceforge had some problems recently, so don't get surprised when you download SharpDeveloper, even if you click on the right link.
However there is of course a technique to hide the source code of beeing dissambled called "obfuscating". When you try to dissamble Code that was modifed by an Obfuscator, it will create an entire mess. There is not much you can do about it.