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