Here are the 2 valid free Eclipse plugins I know about regarding UML class diagrams:
It uses the UML notation to show a graphical representation of existing code that is as accurate and up-to-date as your text editor, while being very easy to use.
The recommended way is a remote Eclipse update site with the following information:
Name: ObjectAid UML Explorer
URL: http://www.objectaid.com
It has a free version which includes class diagrams.
