ChemGlobe: Actinium
   Electronic, thermal, and steric data along with an isotope table.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/89.html

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

EnvironmentalChemistry.com: Actinium
   Atomic structure, properties, and table of nuclides.
   http://environmentalchemistry.com/yogi/periodic/Ac.html

It's Elemental: Actinium
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele089.html

LANL: Actinum
   Basic information and history.
   http://periodic.lanl.gov/elements/89.html

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

Visual Elements: Actinium
   Image, sources, biological role, key isotopes, and oxidation states.
   http://www.chemsoc.org/viselements/pages/actinium.html

Wikipedia: Actinium
   Details the properties of the element, including its history, applications, and characteristics.
   http://en.wikipedia.org/wiki/Actinium