Georgia State University
Georgia State University: Hyper Physics: The Meissner Effect
This site from the Georgia State University provides a good introduction to and illustration of the Meissner Effect, the property of a superconducting material whereby it actively excludes a magnetic field from within its interior.
Wikimedia
Wikipedia: The Meissner Effect
Wikipedia.com offers a wonderful introduction and explanation of the Meissner effect, explaining how this effect qualifies superconduction as a true thermodynamic state of a material.
Other
Colorado Superconductor: The Meissner Effect
This site contains a good conceptual discussion of how Lenz's law affects diamagnetic superconducting materials. Also discusses the Meissner effect and contains a procedure for demonstrating the effect by levitating a magnet over a...