http://www.mathopenref.com/

Math Open Reference provides a list of mathematical topics. Under each topic there are a series of Java Applets. Each Java Applet provides students an opportunity to manipulate a particular shape or situation. The purpose of the site is to explore mathematical relationships by manipulating initial conditions. For example, a student can manipulate a triangle and discover that the orthocenter of the triangle will be outside the triangle if the triangle is obtuse, and inside the triangle if it is acute. I have used this website in my classroom to have students determine various mathematical relationships by exploring and "playing" with the Java Applets. I also have my students reference this website from home when they have a question while their are home studying or working on homework.