Khan Academy: High School Geometry
Khan Academy

Learn high school geometry for free—transformations, congruence, similarity, trigonometry, analytic geometry, and more.

Real World Math: Exploratory Lessons - Pyramid in the Desert
Real World Math

This lesson introduces students to the concept of fractals with a simple fractal curve. 

Illuminations: Blue Squares and Beyond
Illuminations (National Council of Teachers of Mathematics)

This lesson invites students to manipulate two rectangles to create examples of similarity and to study the effects on area ratios.

Illuminations: Go With Green Rectangles
Illuminations (National Council of Teachers of Mathematics)

Students manipulate 2-dimensional rectangles to focus on the relationship between the scale factor and ratio of perimeters of similar rectangles, and the relationship between scale factor and ratio of areas of similar rectangles.

Illuminations: In Your Shadow
Illuminations (National Council of Teachers of Mathematics)

Students extend their knowledge of proportions to solving problems dealing with similarity.