ChemGlobe: Selenium
   Electronic, thermal, and steric data along with an isotope table.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/34.html

ChemicalElements.com: Selenium
   Basic information, atomic structure, and isotopes.
   http://www.chemicalelements.com/elements/se.html

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

It's Elemental: Selenium
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele034.html

LANL: Selenium
   Production, properties, isotopes, and handling.
   http://periodic.lanl.gov/elements/34.html

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

Radiochemistry of Selenium
   Full text of the monograph by V. J. Molinski (Union Carbide Corporation, Mining and Metals Division, Research Center, Tuxedo, New York) and G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
   http://lib-www.lanl.gov/cgi-bin/getfile?rc000058.pdf

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

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

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