GEOMETRY IN ACTION (G.I.A.), from Key Press, is a discovery-oriented book of topics for original sketch-writing on The Geometer's Sketchpad.
Students who create their own sketches really learn geometry!
If your computer has a Java browser, take a look at some of the sketches listed below, especially those marked with asterisks. Then visit Geometry in Action at Key Press.
Triangle Centers (access to lists of classical and recent centers)
Encyclopedia of Triangle Centers