Mobile Phone Plans provides a context for the use of line graphs.
Mobile phone plans: find the best deal 1
Mobile phone plans: find the best deal 2
Mobile phone plans: find the best deal 3
Mobile phone plans: peak and off-peak
Mobile phone plans: one company
Mobile phone plans: several companies

Triathlon helps users explore features of a distance-time graph.
Triathlon: distance-time graphs
Triathlon: triathlete
Triathlon: elite triathletes
Triathlon: the course

NLVM: Algebra manipulatives
Algebra balance scales: negatives
Grapher