Links
Wikipedia — Periodic Table
The Wikipedia article on the periodic table is an excellent reference: history, organization, element properties, and the underlying chemistry.
PTable
An exceptionally rich interactive periodic table with detailed data for every element: electron configurations, isotopes, spectra, physical properties, and more.
Periodic Table
Another comprehensive reference with element data, compounds, and isotope information.
Mendeleev's 1871 Periodic Table (SVG)
The original 1871 table by Dmitri Mendeleev, with German group labels and dashes marking elements he had predicted but which had not yet been discovered. Public domain. Adapted from the Wikimedia Commons SVG by NikNaks.