ArgoUML

Open Source UML Modeling Tool

The self-proclaimed "leading open source UML modeling tool" supports diagram editing for all 9 UML 1.4 diagrams and comes in 10 different languages. Because it's based on Java, it runs on any platform, and it can export diagrams in 6 different file formats - GIF, PNG, PS, EPS, PGML and SVG. Another nice feature is that it supports reverse engineering with Jar/class file importing.