Other
University of Arkansas: Kinetic Theory
A little background and a great Java applet that allows you to see the distribution of particles by their velocity.
University of Virginia
University of Virginia: Kinetic Theory, a Brief Review
The derivation of the Boltzmann distribution in detail. Shows calculations and graphs, with links to animations and video to further explain.
University of Waterloo (Canada)
University of Waterloo: Kinetic Theory of Gases
Takes the Boltzmann distribution mathematics and explains it. Not a simple site, but does provide nice historical links in addition to high-level explanation of the distribution.