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: 
### Method names, fields, etc.

By adding a jar to a project in an IDE, you can usually see methods and field names, but not the detailed implementation. NetBeans can do it, Eclipse probably, IntelliJ probably, etc. You can browse the jar structure directly within the IDE.

### Just the contents

For anything such as viewing the contents, you could use :

* `jar tvf jarfile.jar`
* winzip or any zip tool

### The source code

To access source code, you would use a decompiler such as JAD or one of its frontends or another decompiler. If the code is obfuscated, then ...
I've set the default action in windows to "Open with WinZip". This makes it easy to manage JARs as archives. You can even add/remove files manually.