ChemGlobe: Erbium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Erbium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Erbium: Atomic structure, chemical and physical properties, and table of nuclides.
Erbium: Includes the history of the element, and a data sheet.
It's Elemental: Erbium: Basic physical and historical information.
LANL: Erbium: Basic information and history.
Lenntech: Erbium: Physical data, chemical properties, health and environmental effects.
Visual Elements: Erbium: Image, physical information, key isotopes, and ionisation energies.
WebElements: Erbium: Extensive information on history, uses, occurrence, compounds, and properties.
Wikipedia: Erbium: Properties of the element, including its history, applications, and characteristics.