Chemistry 
General Websites
Chemdex -- Large international directory of chemistry on the Internet
Chemical Sciences (Chemistry) -- Collected links from the Internet Public Library
CHEMINFO -- From Indiana University
Elements
Chemical Elements
It's Elemental
Who discovered the element gold, silver, copper, neon, etc...?
CHEMICAL ELEMENTS
The Berkeley Laboratory Isotopes Project's Exploring the Table of Isotopes
The Elements Song -- from youtube
The Elements Song -- from Wikipedia
Element Tables -- browse by Periodic Table, List by Name, List By Symbol, List By Atomic Number
Specific Aspects
ChemAlliance -- "Regulatory information for the chemical process industries."
ChemFinder Web Server -- Search for information on chemical substances
Molecule of the Month -- Many molecule listings, with substantial background information
Web Elements -- Periodic table, from University of Sheffield, England