PerDoclet is a doclet that generates XHTML documents with UML. A doclet is a software component that alters the output of Sun's javadoc tool. PerDoclet is open source.
PerDoclet have a bunch of advantages over the standard doclet:
2005-08-08: Version 1.1 was released. This version makes PerDoclet open source and distributed under GNU GPL. Download above is source code, you can fetch it compiled too.
2004-05-12: Version 1.0 was released.
Here is a screenshot of a UML class diagram generated by PerDoclet, showing the PerDoclet class itself:

See the full example of documentation for PerDoclet, including private methods. The full example was generated with this command:
javadoc -private -doclet PerDoclet PerDoclet.java
PerDoclet has some doclet-specific javadoc parameters. Execute javadoc -doclet PerDoclet to see them.
Note that if you want private members to be shown in the UML class diagram, you must specify javadoc's -private parameter.