Introduction/Overview/Beginner
Chemistry Wiki: This page contains a brief overview of delocalized electrons. This page is better organized than the Wikipedia page on delocalization.
Wikipedia: Delocalization: This is the Wikipedia page for delocalization.
Wikipedia: Conjugation: This is the Wikipedia page for conjugation.
Master Organic Chemistry
Intermediate/More Detail
Delocalization and π Bonds: This link goes into detail about stability, resonance, and molecular orbital theory of adjacent p orbitals.