Calcium

ChemGlobe: Calcium
Electronic, thermal, and steric data along with an isotope table.
http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/20.html

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

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

It's Elemental: Calcium
Basic physical and historical information.
http://education.jlab.org/itselemental/ele020.html

LANL: Calcium
History, sources, properties, uses, and compounds.
http://periodic.lanl.gov/elements/20.html

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

Visual Elements: Calcium
Image, animation, source, uses, and physical information.
http://www.chemsoc.org/viselements/pages/calcium.html

WebElements: Calcium
Extensive information on history, uses, occurrence, compounds, and properties of the element.
http://www.webelements.com/webelements/elements/text/Ca/key.html

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