MollyCule is a completely free Open Source Chemistry visualisation program for MacOS, Linux and Windows, based on Java and Java3D. I wrote it back in 2001, and thanks to incredible progress in 3D graphics technology, it's faster than ever.
It's designed to be a friendly, easy to use program to help ordinary people who are learning about Chemistry. The interface is extremely intuitive and user-friendly. It's ideal for schoolteachers, schoolchildren and 1st/2nd year undergraduates.
You may find it useful for easily exploring and visualising the structure of simple molecules, making pictures/screenshots for school/university chemistry reports, or providing an animated screensaver and backdrop for school or university chemistry presentations. However, it's not the right tool if you're looking to do e.g. energy/distance calculations or other serious matters.
Free to use for Mac, Linux and Windows - click here.