Oxygen

ChemGlobe: Oxygen
Electronic, thermal, and steric data along with basic information.
http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/8.html

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

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

Especiál Gas Inc.: Oxygen
Numeric data, uses, and general information.
http://www.c-f-c.com/specgas_products/oxygen.htm

It's Elemental: Oxygen
Basic physical and historical information.
http://education.jlab.org/itselemental/ele008.html

LANL: Oxygen
History, sources, forms, compounds, and isotopes.
http://periodic.lanl.gov/elements/8.html

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

Visual Elements: Oxygen
Animation, uses, physical information, and key isotopes.
http://www.chemsoc.org/viselements/pages/oxygen.html

WebElements: Oxygen
Information on history, uses, occurrence, compounds, and properties of the element.
http://www.webelements.com/webelements/elements/text/O/key.html

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