ChemGlobe: Erbium
   Electronic, thermal, and steric data along with an isotope table.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/68.html

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

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

It's Elemental: Erbium
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele068.html

LANL: Erbium
   Basic information and history.
   http://periodic.lanl.gov/elements/68.html

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

Visual Elements: Erbium
   Image, physical information, key isotopes, and ionisation energies.
   http://www.chemsoc.org/viselements/pages/erbium.html

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