Starting from version 7 it will cost you 35$ for a perpetual license. Apparently they can afford working on this for free. You can read more on this decision here.
What is .NET Reflector?
For those of you who doesn't know the tool - it's a very good class browser, analyzer and decompiler for .NET. I use it quite often to decompile dlls if I need to check what's actually in there.
Can you recommend a similar free tool?