Avogadro
Avogadro is a free, fantastically easy to use molecule editor. Because this editor is cross platform and light on computer resources, you know you can run it when and where you need it. With multi-threaded rendering and computation, however, Avogadro can draw on whatever resources it needs to get you blazing performance. I liken the difference between Avogadro and competing software to that between Google Sketchup and the various CAD programs. The real power comes from the plugin architecture for developers and scientists to tailor Avogadro to their specific needs and designs. For example, Avogadro can perform a global search for stable crystal structures using evolutionary techniques.
It can even work with DNA! Other features include: Open Babel import of chemical files, input generation for multiple computational chemistry packages, crystallography, biomolecules, and more being developed as we speak.
