Network visualizations:

"That is a very graphic analogy which aids understanding wonderfully while being, strictly speaking, wrong in every possible way," said Ponder [Stibbons].
—Making Money, T. Pratchett.

Still, people seem to enjoy looking at large networks, and we'll build up a supply over the semester.

Amazon's recommender system:

Amazon recommender system.

Flight patterns over the United States:

By Aaron Koblin. See here for more images and information.

Network of high school romantic ties:

Chains of affection: The structure of adolescent romantic and sexual networks.
P. S. Bearman, J. Moody, K. Stovel.
American Journal of Sociology, Vol. 100, 44–91.

Chains of affection.