ChemGlobe: Cobalt: Electronic, thermal, and steric data, along with isotopes table.
ChemicalElements.com: Cobalt: Basic information, atomic structure, and isotopes.
Cobalt: Data tables and historic information.
EnvironmentalChemistry.com: Cobalt: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Cobalt: Basic physical and historical information.
LANL: Cobalt: History, sources, properties, and uses.
Lenntech: Cobalt: Physical data, chemical properties, health and environmental effects.
Radiochemistry of Cobalt: Full text of the monograph by L.C. Bate (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
USGS Minerals Information: Cobalt: Statistics and information on the worldwide supply, demand, and flow of the element (PDF format).
Visual Elements: Cobalt: Image, general and physical information, source, uses, biological role, key isotopes, and ionisation energies.
WebElements: Cobalt: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Cobalt: Properties of the element, including its history, applications, and characteristics.