This is THE ultimate guide to all chemical terminology, compiled by none other than The International Union of Pure and Applied Chemistry (IUPAC).
This site has been around since 1993, has undergone radical changes since, and is still undergoing continual development. It is now one of the most popular and reliable online Periodic Tables.
"The RSC is the largest organisation in Europe for advancing the chemical sciences. Supported by a worldwide network of members and an international publishing business, our activities span education, conferences, science policy and the promotion of chemistry to the public."

The RSC site has many resources for both teachers and students, and for chemical education in general. If you are into Chemistry, this is the site you must visit.
This site, written by Jim Clark, provides an excellent guide to 'A' Level Chemistry. The various topics are well-organised, and his explanation is clear and concise.
Besides detailed revision notes on this site, there are exercises which serve as a quick check to your understanding. The only drawback is that the website is cluttered with advertisements. Doc Brown also writes notes for Physics and Biology.
The Wired Chemist is an online collection of scientific resources, including databases, tutorials and animations. Its tutorials cover both theoretical concepts as well as laboratory techniques, and some of the animations featured are interactive and require input from the user.
This is an excellent online resource covering general, inorganic, organic, physical and interdisciplinary areas of chemical sciences. Although the content is pitched above the requirements of Higher 2 Chemistry Syllabus, given the current demands of the H2 Chemistry paper, reading beyond the standard text will stand students in good stead.
This virtual textbook is owned by Michigan State University Emeritus Professor William H Reusch. The contents may be too advanced for most 'A' Level chemistry students, but sections like Structure & Bonding, Intermolecular Forces, Reaction Mechanisms and Spectroscopy can serve as a reference and resource for practice problems for higher-ability students taking the Higher 3 Pharmaceutical Chemistry.
This is a web resource developed by ChemAxon which uses ChemAxon's Name to Structure parsing to identify chemical structures. I find this resource very useful as I can find information on IUPAC names, isoelectric points, pKa values, stereoisomers etc. of molecules with a single click.

Chemistry In Action!

The following excerpt, taken from the American Chemical Society - Education site, effectively sums up the importance of chemistry in the world around us.

"Chemistry Is Everywhere. Everything you hear, see, smell, taste, and touch involves chemistry and chemicals (matter). And hearing, seeing, tasting, and touching all involve intricate series of chemical reactions and interactions in your body. With such an enormous range of topics, it is essential to know about chemistry at some level to understand the world around us ..."
This page provides links to videos which demonstrate how common chemistry laboratory techniques like volumetric analysis, distillation and refluxing a reaction are performed. These videos are made by staff and students of Massachusetts Institute of Technology as part of their open courseware.
This software, by ACD/Labs, is a drawing package that allows you to draw chemical structures. It comes as a freeware version, as well as a commercial version.

For something that is available free, the freeware version is surprisingly packed with many useful features for teachers like myself. In addition to chemical structures, I also use it to draw atomic orbitals, dot-and-cross diagrams and lab equipment. Unfortunately, it only comes in PC version. I tried running it on my MacBook Pro once via a virtualisation software, but gave it up when it didn't seem stable.
This is also a chemical editor, developed by ChemAxon, for drawing chemical structures. Unlike ChemSketch, this one runs on Mac. I'm not particularly fond of it though, as I can't cut and paste the structures I drew onto Microsoft documents. I only use this when I have no access to a PC or laptop running on Microsoft OS.

ChemAxon also developed, a public web resource which uses ChemAxon's Name to Structure parsing to identify chemical structures. I find this resource very useful as I can find information on IUPAC names, isoelectric points, pKa values, stereoisomers etc. of molecules with a single click.