Javadoc doclet which generates HTML report of differences between two APIs. [Free]

Plug-in for Eclipse enables explore Java code visually allowing see its structure and the links (calls, accesses, etc) between its different pieces. [Freeware]

Class and package browser for quickly finding a class, its source, its methods, fields, constructors... and to be able to explore packages. Completion, class homonyms and disassembly may be enabled. [Open Source, LGPL]

A Java class browser and project manager for Emacs. [Open source, GPL]

a combination of Java quick reference, class hierarchy browser, documentation generator and browser for Win32 platform [Commercial].

A tool for collection, correlation, and visualizing software metrics. [Open source, MITL]

Get constructor, field, method information with free search for methods and option to view javadocs at method with your preferred browser. Configured by XML document. [Open Source, LGPL]

A tool which converts a bunch of Java Source Code into a colourized and browsable HTML representation. [Freeware]

A tool for visualization (viewing, searching, comparing) and manipulation (modifying, refactoring) of Java Class files. The developers offer an overview over features, FAQ, download, documentation and tutorial. [Open Source, LGPL]