The American Chemical Society is a great place to find a lot of different topics on the science and chemistry scene. Check out the Chemcenter!

This is a Comic Book Periodic Table that will blow you away. If you check just one of these links, check out this one because you will enjoy the work done on this link's website.

Kiwi Web is a nice chemistry website that defines and talks about the different types of chemistry and other interesting tidbits.

Stoichiometry Problems and Practice: In depth tutorials and practice quizzes to help you master moles, grams, molar mass, and more.

Webelements is a highly interactive periodic table page. It has an ample amount of element information.

Yogis has a great chemistry dictionary. But that is not all, it has an interactive periodic table and molar calculations with a javascript calculator.