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...?
The Berkeley Laboratory Isotopes Project's Exploring the Table of Isotopes
The Elements Song -- from youtube
Element Tables -- browse by Periodic Table, List by Name, List By Symbol, List By Atomic Number
Web Elements -- Periodic table, from University of Sheffield, England
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