A PowerPoint Presentation with everything students need to know about circles! http://path.btcs.org/users/smithd/circles.zip
Interactive math sites for geometry. http://matti.usu.edu/nlvm/nav/category_g_3_t_3.html
Isometric DrawingTool - This applet allows you to draw on isometric dot paper. Good for 3-D representations. http://illuminations.nctm.org/index.html#ftmathlets/isometric/