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