Pericyclic and other Orbital Symmetry-Controlled Reactions
Prof. Howard Black - Eastern Illinois University
Pericyclic Reactions  the great five-lecture course at Imperial College by Dr. Henry Rzepa, complete with 40 problems.  Theory is rigorous but not difficult - clearly presented with lots of diagrams, colored models, etc.
Pericyclic Reactions  by Prof. Bill Reusch (Michigan) part of an amazing virtual organic chemistry textbook.  Really good.
Pericyclic Reactions   undergrad-targeted site from U. of S. Maine - great color, very conceptual and easy to understand. If you're feeling lost, this is a great place to go!
Pericyclic Reactions  from Yale - Very clear, short, accurate summary.  Beautiful diagrams, conceptual material that's very easy to follow - particular focus on Woodward-Hoffman rules and their application.   Extremely well done.
Pericyclic Reactions Review (Magid) - eight-page worksheet on various aspects of or bital symmetry.  Much better if you can print it in color.
Pericyclic Reactions: A six-lecture series by Prof. Roger Alder (Bristol):
Ring Synthesis(1 & 2)      Diels-Alder Rxns. (3 & 4)     Others (5 & 6)
Orbital Symmetry Control of Pericyclic Reactions (Magid) - tremendous, carefully organized collection of articles. Awe-inspiring, at 47 pages long!
Definitions of Orbital Symmetry Terms Dr. Ronald Magid (U. of KY)
Journal List  and Lit. Refs for Orbital Symmetry Prof. Magid - somewhat dishevelled but huge list of references to primary literature, organized by type of pericyclic reaction.
Theoretical Chemistry [don't run away screaming!] Click on "Computational Organic Chemistry"; there are four useful links, including cycloaddition.  Other features include basic quantum mechanics (incl. computational methods), electromagnetic properties, and statistical mechanics.