update 11 May 2011: Unfortunately Reflector is no longer free since March 2011. Luckily JetBrains created a new decompiler that is free. You can find more information here:
http://lawo.wordpress.com/2011/05/11/tools-the-free-decompiler/ or on the JetBrains website.
One of the most useful and indispensable pieces of software in my entire .NET toolkit. During SharePoint programming I use it mostly for lookup the full assembly names of dll’s. It’s also handy when you want to disassemble .NET code.
Most people know this tool as the Lutz Roeder .NET Reflector. Nowadays the development is in the hands of the company Red Gate. Lutz has reached an agreement with Red Gate so they continue the development and Lutz can focus on other pet projects.
MSDN magazine named it “one of the 10 must-have tools every developer should download now.