This WebElements site provides a great deal of elemental information. There are audio files demonstrating the correct pronunciation of the element name, pictures showing the electron shells, and information on isotopes, properties and more.

A nice, clear site, containing a good deal of elemental information, including atomic weight, density, boiling point, isotopes and more. An image shows the electron energy levels for the element.

A nice summary of the properties of magnesium. Very easy to use.

Here you can find a detailed overview of aluminum. Content includes a focus on aluminum's chemical properties, occurrence in nature, uses, and production.