Chemical Information Sources Wiki
The Chemical Information Sources Wiki is a guide to the many sources of reference materials available for those with questions related to chemistry. The material is based on an undergraduate course offered for many years in the Indiana University Department of Chemistry by Dr. Gary Wiggins.
http://cheminfo.informatics.indiana.edu/cicc/cis/index.php/Main_Page

ChemWiki
The ChemWiki is a new component of Mark Winter's www.WebElements.com site. You will find pages about many aspects of chemistry here - and if you like you can edit them, and add new pages as well.
http://wiki.webelements.info/

Encyclopedia 4U - Chemistry
A very detailed encyclopedia covering all aspects of chemistry that would be used in freshman level chemistry classes.
http://www.encyclopedia4u.com/c/chemistry.html

Encyclopedia of Chemistry
Growing encyclopedia with articles on chemical reactions, experimental chemistry, quantum chemistry, inorganic and organic chemistry.
http://scienceworld.wolfram.com/chemistry/

Kirk-Othmer Encyclopedia of Chemical Technology
Highly regarded source of information in applied chemistry and chemical technology. About half the articles focus on industrially important chemical substances or materials, their properties, manufacture, and industrial and economic significance. Other articles cover chemical processes and engineering principles, current research, and emerging technologies; environmental, safety, and health topics; and selected chemistry fundamentals.
http://www3.interscience.wiley.com/cgi-bin/mrwhome/104554789/HOME

Little Encyclopedia of Mass Spectrometry
The Little Encyclopedia of Mass Spectrometry was developed by the Mass Spectrometry Group of the University of Heidelberg, and is intended to serve as a fast source of basic knowledge of mass spectrometry. Explanations are given in a tutorial-like form aided by diagrams and links to the selected topic areas are provided for further explanation.
http://www.rzuser.uni-heidelberg.de/~bl5/encyclopedia.html

Wikipedia
Perhaps the best place to start is the entry for "Chemistry" in the Wikipedia, which is an open source online encyclopedia. The entry provides a very useful set of definitions for key terms in almost all areas of chemistry. As a reader, if you see something missing you can add it, or if there are mistakes, you can edit them.
http://en.wikipedia.org/wiki/Chemistry/