Chemicool
Chemicool: Helium
This site from Chemicool provides a set of tables summarizing a great deal of information on helium and its properties.
Florida State University
Florida State University: Magnet Lab: Dilution Refrigerators
Dilution fridges owe their cooling power to the incredible element helium. This animation illustrates how dil fridges exploit the element's properties to make things very, very cold.
Georgia State University
Georgia State University: Hyper Physics: Leptons
This site has a definition and properties of Leptons, the "Basic building blocks of matter." There are links to table of properties and more information.
Other
Virtual Chembook: Density Applications With Gases
DENSITY is a physical property of matter, as each element and compound has a unique density associated with it. Density defined in a qualitative manner as the measure of the relative "heaviness" of objects with a constant volume. For...
Khan Academy
Khan Academy: Ideal Gases in Medicine
This passage tests your knowledge on the kinetic molecular theory of gases using the five-question quiz.