ChemGlobe: Selenium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Selenium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Selenium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Selenium: Basic physical and historical information.
LANL: Selenium: Production, properties, isotopes, and handling.
Lenntech: Selenium: Physical data, chemical properties, health and environmental effects.
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]
Selenium: Data tables and historic information.
USGS Minerals Information: Selenium: Statistics and information on the worldwide supply, demand, and flow of the element (PDF format).
Visual Elements: Selenium: Image, general and physical information, source, uses, key isotopes, and ionisation energies.
WebElements: Selenium: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Selenium: Properties of the element, including its history, applications, and characteristics.