http://hotmath.com/util/hm_flash_movie_full.html?movie=/learning_activities/interactivities/hoptoit.swf

http://www.emaths.co.uk/tutorials/Brackets2/Multiplying%20out%20two%20Brackets/Presentation_Files/index.html

http://www.mathsonline.co.uk/nonmembers/gamesroom/sims/archi/data.html

http://phet.colorado.edu/index.php