ChemGlobe: Rubidium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Rubidium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Rubidium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Rubidium: Basic physical and historical information.
LANL: Rubidium: Sources, properties, isotopes, and uses.
Lenntech: Rubidium: Physical data, chemical properties, and health effects.
Radiochemistry of Rubidium: Full text of the monograph by G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
Rubidium: Data tables and historic information.
Visual Elements: Rubidium: Image, source, uses, physical information, and key isotopes.
WebElements: Rubidium: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Rubidium: Properties of the element, including its history, applications, and characteristics.