JGraphT - a free Java library of graph-theory objects and algorithms
 

JGraphT is a free Java class library that provides mathematical
graph-theory objects and algorithms. JGraphT supports a rich gallery
of graphs and is designed to be powerful, but at the same time easy to
use.

JGraphT graphs can be visualized -- see a demo at:
http://jgrapht.sourceforge.net/visualizations.html

JGraphT comes with:
* the complete source code (under GNU LGPL)
* comprehensive javadocs
* free support

Visit http://jgrapht.sourceforge.net/ to learn more and download.