Question: What would be the easiest way to view classes, methods, properties, etc. inside a jar file?
I'm looking for something equivalent to the very useful  *Lutz Roeder .NET Reflector*  - for Java

Answer: 
Eclipse 3.4 JDT

It is not the quickest way because you have to drag it into your eclipse first.
But you will have full java class browsing, even with decompile enabled.
You could try [JarSpy](http://sourceforge.net/project/showfiles.php?group_id=42094). There is an IDEA plugin version of it that I use.