ChemGlobe: Sulfur
   Electronic, thermal, and steric data along with basic information.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/16.html

ChemicalElements.com: Sulfur
   Basic information, atomic structure, and table of isotopes.
   http://www.chemicalelements.com/elements/s.html

EnvironmentalChemistry.com: Sulfur
   Atomic structure, chemical and physical properties, and table of nuclides.
   http://environmentalchemistry.com/yogi/periodic/S.html

It's Elemental: Sulfur
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele016.html

LANL: Sulfur
   History, properties, production, and uses.
   http://periodic.lanl.gov/elements/16.html

Lenntech: Sulfur
   Physical data, chemical properties, health and environmental effects.
   http://www.lenntech.com/Periodic-chart-elements/S-en.htm

Radiochemistry of Sulfur
   Full text of the monograph by G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
   http://lib-www.lanl.gov/cgi-bin/getfile?rc000050.pdf

Sulphur versus Sulfur
   Some thoughts on the history of the spelling of the name of element 16: 'sulfur' vs 'sulphur'.
   http://www.quinion.com/words/topicalwords/tw-sul1.htm

USGS Minerals Information: Sulfur
   Statistics and information on the worldwide supply, demand, and flow of the element (PDF format).
   http://minerals.usgs.gov/minerals/pubs/commodity/sulfur/

Visual Elements: Sulfur
   Image, general and physical information, source, uses, key isotopes, and ionisation energies.
   http://www.chemsoc.org/viselements/pages/sulfur.html

Wikipedia: Sulfur
   Properties of the element, including its history, applications, and characteristics.
   http://en.wikipedia.org/wiki/Sulfur