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