Thallium

ChemicalElements.com: Thallium - Basic information, atomic structure, and isotopes.
http://www.chemicalelements.com/elements/tl.html

Lenntech: Thallium - Physical data, chemical properties, health and environmental effects.
http://www.lenntech.com/Periodic-chart-elements/Tl-en.htm

It's Elemental: Thallium - Basic physical and historical information.
http://education.jlab.org/itselemental/ele081.html

LANL: Thallium - History, sources, properties, handling, and uses.
http://periodic.lanl.gov/elements/81.html

Wikipedia: Thallium - Properties of the element, including its history, applications, and characteristics.
http://en.wikipedia.org/wiki/Thallium

Visual Elements: Thallium - Image, source, uses, physical information, and key isotopes.
http://www.chemsoc.org/viselements/pages/thallium.html

USGS Minerals Information: Thallium - Statistics and information on the worldwide supply, demand, and flow of the element (PDF format).
http://minerals.usgs.gov/minerals/pubs/commodity/thallium/

EnvironmentalChemistry.com: Thallium - Atomic structure, chemical and physical properties, and table of nuclides.
http://environmentalchemistry.com/yogi/periodic/Tl.html

WebElements: Thallium - Extensive information on history, uses, occurrence, compounds, and properties.
http://www.webelements.com/webelements/elements/text/Tl/key.html

ChemGlobe: Thallium - Electronic, thermal, and steric data along with an isotope table.
http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/81.html