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