Because of the fact that the most valueable tool in the SharePoint-World beside SharePoint Manager (I mean our beloved .net Reflector) is no longer free and becomes unsusable after a few weeks until it looks for updates, some resourceful developers started writing an open source equivalent, which can be found at:
http://wiki.sharpdevelop.net/ILSpy.ashx
I downloaded and tested it already and I think, it's really advanced for the short time elapsed since its development started in february 2011. The look and feel is very similar to .net Reflector and you definitely should give it a try.