OnlineMathLearning.com
Online Math Learning: Probability: Complementary Events
An explanation of complementary events in probability along with several examples and step-by-step solutions.
Shodor Education Foundation
Shodor Interactivate: Lesson: Ideas That Lead to Probability
This lesson plan sets the ground work for an introduction to and then exploring the concepts that are the foundation for probability. Lots of resources are available for teachers and young scholars at this website.