Jaree is a little application that wraps a user interface around the jar and javap tools to let you see the contents of Java JAR files. It shows you the hierarchical contents of the packages, and double-clicking on a class shows you its members and methods. These can be copied to the clipboard, and one nicety is that you can also copy their internal type signatures which is useful when writing JNI code.
The 1.1d6 version is a development build of a rewrite in Java instead of REALbasic. It is experimental in the sense that it attempts to produce the same feel and quality as the REALbasic version. It is pretty much complete and fixes a few problems of the 1.0 version.