Jmol is free software for displaying interactive 3D molecules. It is designed as a component that can be used as a standalone application, as an applet within the context of a web page, or as a display subsystem within a more sophisticated software package. Because it is written in Java and requires no special graphics hardware, Jmol runs on all major operating systems and web browsers. Jmol has been developed as a collaborative project using standard open-source software development methodologies and procedures. All source code is publicly available under the GNU licenses, facilitating experimentation, testing, public contributions, and peer review. Rendering representations can be controlled using an extended version of the RasMol scripting language. The Jmol applet allows web content developers to display interactive 3D representations of molecules within web pages, providing an upgrade path for users of the Chime web browser plugin. New capabilities are being added to Jmol on an ongoing basis. As Jmol has matured over the past few years, it has become an increasingly popular component in computer based molecular visualization. We expect Jmol's popularity to continue to grow.