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

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

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

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

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

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/

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

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