"The Royal Society of Chemistry's interactive periodic table features history, alchemy, podcasts, videos, and data trends across the periodic table." (website description)
Based on Wolfram Mathematica's ElementData. Links to pictures (including minerals), technical & isotope data. Technical data link provides graphical comparisons with other elements.
Tabbed browsing of elements (history, biology, thermochemistry), compounds (binary and complexes), periodicity(physical properties), chemistry books, etc.
A periodic table with links to descriptions, physical properties and recent literature references. Each element's page has a listing of related commercial compounds with links to descriptive pages.