ChemGlobe: Manganese: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Manganese: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Manganese: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Manganese: Basic physical and historical information.
LANL: Manganese: Sources, properties, uses, and handling.
Lenntech: Manganese: Physical data, chemical properties, health and environmental effects.
Manganese: Data tables and historic information.
Radiochemistry of Manganese: Full text of the monograph by R. P. Schuman (Dept. of Chemistry, Robert College, Bebek P.K.8, Istanbul, Turkey). [PDF]
USGS Minerals Information: Manganese: Statistics and information on the worldwide supply, demand, and flow of the element (PDF format).
Visual Elements: Manganese: General and physical information, source, uses, biological role, key isotopes, and ionisation energies.
WebElements: Manganese: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Manganese: Properties of the element, including its history, applications, and characteristics.