ChemGlobe: Potassium: Electronic, thermal, and steric data along with basic information.
ChemicalElements.com: Potassium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Potassium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Potassium: Basic physical and historical information.
LANL: Potassium: Sources, uses, properties, and isotopes.
Lenntech: Potassium: Physical data, chemical properties, and health effects.
Potassium: Data tables and historic information.
Radiochemistry of Potassium: Full text of the monograph by William T. Mullins and G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
Visual Elements: Potassium: Image, animation, source, uses, physical information, and key isotopes.
WebElements: Potassium: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Potassium: Properties of the element, including its history, applications, and characteristics.