ArgoUML is licensed under the BSD license which also allows extensions to be commercialized.
The company Gentleware offers such an extension under the name Poseidon for UML.
A Community Edition is offered free of charge.

To be effective agile modelers should know a wide variety of modeling techniques. Unfortunately this is easier said than done. This page links to summary descriptions of a wide variety of modeling artifacts. Each page describes the artifact, provides a

Violet, an open-source, cross-platform UML editor.

List of UML tools by price

freeware and shareware uml tool

ESS-MODEL: The fastest and easiest to use UML reversing tool on the market!

Examples and tutorials for design patterns

UML models of various design patterns

A very handy tutorial and documentation about UML

The JDK Class Documentation Project has the goal, to chart the standard packages of Java. Presently there are 233 class diagrams, which describe most of the classes of the JDK 1.4.2. The diagrams are organized in a folder like manner, which corresponds to