University of Saskatchewan (Canada)
University of Saskatchewan: Triangles
This site from the University of Saskatchewan provides an explanation of similar triangles, the Pythagorean theorem, and the distance between arbitrary points in a plane. Example problems for each of these concepts are provided.
Khan Academy
Khan Academy: Least Common Multiple
Find the lcm (least common multiple) of pairs of integers. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.