ChemGlobe: Magnesium
   Electronic, thermal, and steric data along with basic information.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/12.html

ChemicalElements.com: Magnesium
   Basic information, atomic structure, and table of isotopes.
   http://www.chemicalelements.com/elements/mg.html

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

It's Elemental: Magnesium
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele012.html

LANL: Magnesium
   History, properties, uses, compounds, and handling.
   http://periodic.lanl.gov/elements/12.html

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

Radiochemistry of Magnesium
   Full text of the monograph by A. W. Fairhall (Dept. of Chemistry, University of Washington, Seattle, Washington). [PDF]
   http://lib-www.lanl.gov/cgi-bin/getfile?rc000047.pdf

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

Visual Elements: Magnesium
   Image, general and physical information, source, uses, key isotopes, and ionisation energies.
   http://www.chemsoc.org/viselements/pages/magnesium.html

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