Periodic Table lots of information about each element!
NIST Chemistry Webbook (good source of thermodynamic, spectroscopic data)
Fundamental Constants: http://physics.nist.gov/cuu/Constants/index.html
CRC Handbook of Chemistry and Physics on-line
International Critical Tables on-line
Chart of Nuclides from Brookhaven National Lab
Table of Nuclides from Lawrence Berkeley Lab (masses, decay modes, half-lives, etc.)
Dating the Shroud of Turin (reprint of 1989 Nature paper)
Ionising Radiation: Quantities and Units (from the UK National Physical Lab)
Many chemistry links: http://www.liv.ac.uk/Chemistry/Links/
Molecules to Go (Database of biological molecules; can be viewed, rotated in 3D)
NIOSH Pocket Guide to Chemicals
(safety information)Chemical Heritage Foundation (History of Chemistry)
Columbia Chemistry Library
Just for FUN: http://www.uky.edu/Projects/Chemcomics/
Barnard chemistry department home page: Barnard Chemistry Department