ChiMeraL 
(http://www.ch.ic.ac.uk/chimeral/)

This chemical markup language site is important, it comes from one of the pioneers of the chemical internet, Imperial College London's Henry Rzepa, and discusses a web-based approach to chemistry using eXtensible Markup Language and its sibling CML. Such efforts will provide transparency, universality and most crucially, chemical savvy on the internet and open up communication channels between chemists. Coming to a chemical web near you soon.

Extended Periodic Table 
(http://www.apsidium.com/ext_pt/expertab.htm)    

Lanthanides? Check! Actinides? Check? Superactinides? Err, check!, Ekasuperactinides? Huh?

If you're looking for more than the standard ninety-odd elements and want to stretch into the outer reaches of elemental discoveries, try this site for elements beyond 118. Element 201 for instance is binilunium, with an atomic weight of 556 and electron shells as follows, 2,8,18,32,50,50,32,8,1. No melting or boiling point is recorded. Fascinating stuff!

ChemTeam
(http://dbhs.wvusd.k12.ca.us/ChemTeamIndex.html)    

The ChemTeam provides a useful study resource for the various chemistry topics a high school student might encounter. You can find everything from acids and bases to redox and from equilibria to radioactivity. An old-fashioned looking site but then as they said in the good old days "it's content and context that matter."

Chemicalc 
(http://www.chemicalc.com/)    

The home of the "chemical calculator" with its built in interactive periodic table helps students - and even teachers - to quickly and accurately complete typical chemical calculations, such as balancing equations, exploring reaction stoichiometry, unit and mole conversions, working out elemental compositions and mass spec calculations, and figuring out empirical formulae. A downloadable demo is available with tutorial sessions.