Just a quick note, if you want a decompiler for .NET, JetBrains, publishers of Resharper, my favorite Visual Studio add-in utility have released theirs and it's called dotPeek at version 1.0 and it's free.
My own experience of decompilers are that you think they're useful but in reality, not that much. A nice to have in some fairly rare cases. I have heard of them being used to try and regenerate lost source code.
I have lost source code before, a single file back in the bad old days before version control systems existed (on 8 bit micros!) and it was a bit of a nightmare. Fortunately though, because I had done the work fairly recently I was able to recreate it within a day. Mind you it was a very long day and not an experience not to be repeated but I was very lucky.
- Permanent link added for dotPeek to Useful .NET Tools and Utilities