Jmol is a Free, Open Source molecule viewer and editor. It is a collaboratively developed visualization and measurement tool for chemical scientists. Jmol is an active project, and there are new features being added to it on a daily basis. Users are encouraged to modify it to fit their needs and to contribute their changes to the project.
Teriodic is a training tool for the periodic table of the elements. Starting from 1 (hydrogen), it quizzes the abreviations of each element in numerical order. If you make a mistake, it shows you the next five elements and lets you start again from hydrogen. Knowing the periodic table makes it possible to estimate what chemicals look like and how they react.