In simple terms, .NET Reflector is… a must have. It was developer by Lutz Roeder back in 2000 before he went on to join Microsoft. Red Gate have announced that they have acquired it and the good news is a free version will continue to be available (Thanks to Ian for the heads up)

There is more detail over at http://www.simple-talk.com/opinion/opinion-pieces/the-future-of-reflector-/