SwingLabs is an Open Source laboratory for exploring new ways to make Swing applications easier to write, with improved performance and greater visual appeal. It is an umbrella project for various Open Source initiatives sponsored by Sun Microsystems and is part of the java.net community. Successful code and concepts may be migrated to future versions of the Java™ platform.

SwingLab's SwingX provides a powerful suite of desktop components for all your Swing projects. Available under the LGPL license, the SwingX project is the perfect match for both your professional and personal projects.

Links: Website, Screenshots

I am impressed by the quality and the selection of the components. Highly recommended.