ChemGlobe: Silicon: Electronic, thermal, and steric data along with basic information.
ChemicalElements.com: Silicon: Basic information, atomic structure, and table of isotopes.
EnvironmentalChemistry.com: Silicon: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Silicon: Basic physical and historical information.
LANL: Silicon: History, sources, uses, and properties.
Lenntech: Silicon: Physical data, chemical properties, and health effects.
Radiochemistry of Silicon: Full text of the monograph by David R. Schink (Isotopes, a Teledyne Company, Palo Alto, California). [PDF]
Silicon: Data tables and historic information.
Silicon Chemistry Primer: Review of silicon and silicone chemistry including properties and types of materials that can be made.
USGS Minerals Information: Silicon: Statistics and information on the worldwide supply, demand, and flow of the element (PDF format).
Visual Elements: Silicon: General and physical information, source, uses, biological role, key isotopes, and ionisation energies.
WebElements: Silicon: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Silicon: Properties of the element, including its history, applications, and characteristics.