Graphics

G
Generic 2D graphics library for Java. [Open source, LGPL]
http://geosoft.no/graphics/

GEF - Graph Editing Framework
Class library for constructing graph/diagram editing applications. [Open Source, Apache]
http://gef.tigris.org/

GINY (Graph INterface librarY)
Implements a very innovative system for sub-graphing and allows for stunning visuals and it provide a number of layout algorithms. [Open source, LGPL]
http://csbi.sourceforge.net/

HyperGraph
A Swing based API to visualise graphs using hyperbolic geometry (hyperbolic trees). Example applet provided. [Open source, GPL]
http://hypergraph.sourceforge.net/

ILOG JViews
Library and components based on Java2D for maps, diagrams, graph layout, gantt chart, charts, editors and workflow. For client or server side graphics, including SVG.
http://www.ilog.com/products/jviews/

Image Toolz
Manipulation methods for inversion, rotation, shifting, adding, loading, saving and creating images. (Open Source)
http://www.bluering.nl/imagetoolz/index.html

Java Image Visualization
Applets, beans and applications. [Freeware]
http://my.execpc.com/~larryhr/viewer/imageviewer.html

Java Imaging and Graphics Library
Provides classes for data storage, image processing, image I/O and image display [Open Source, GPL].
http://rivit.cs.byu.edu/jigl/v1_5/index.htm

Java-Gnome
Java GTK+ and Gnome bindings using JNI. [Open Source, LGPL]
http://java-gnome.sourceforge.net/

JDrawing
A Swing component for displaying vector graphics and animation. It can deal efficiently with very large vector images e.g. geographic maps or electronic circuits.
http://jdrawing.sourceforge.net/

JGo graphical object editor
A library for graphs, diagrams and networks, with an option for automatic layout by Northwoods Software. [Commercial]
http://www.nwoods.com/go/jgo.htm

JGraph
Graph visualization component for Swing. Includes tutorial, FAQ, and user guide. [Open Source, LGPL]
http://www.jgraph.com/

JInsight.com
Offers various class libraries including "JetChart Library", which displays a variety of types of charts. [Commercial]
http://www.jinsight.com/

JIU - Java Imaging Utilities
This library offers functionality to load, analyze, process and save pixel images. [Open Source, GPL]
http://schmidt.devlib.org/jiu/

OpenJGraph
Various graph-drawing and traversal algorithms. Support for user interaction such as adding and removing vertices. [Open Source, GPL]
http://openjgraph.sourceforge.net/

PJA Toolkit
For generating images on servers, without the need for an X server. [Open Source, GPL]
http://www.eteks.com/pja/en/

VirtualMachine.com
Utility class libraries: VMMemoryWatcher library that visually track memory changes in Java Virtual Machine; Flow monitor library that visually track process flow between containers. [Open source]
http://www.virtualmachine.com/java/

XColor
Subclass of java.awt.Color that knows 752 colors by name - can be substituted for wherever java.awt.Color is used in programs. [Freeware]
http://www.darwinsys.com/freeware/XColor.jar

Zooki Technologies
ExplicitLayout layout manager (Commercial) and EventInitializer to replace your anonymous inner classes (BSD).
http://www.zookitec.com/