Geometer's Sketchpad is a tool that may be used in my classroom from time to time. It is interactive geometry software that can be used to create java html files for posting online as I have with a java kaleidoscope here. It can also be used to construct lessons online. This sample lesson that I have constructed goes over the foundation of the distance formula, linking it to the Pythagorean Theorem. The foundation for this activity was pulled from this purplemath.com lesson.  The SOLs covered by this activity are listed here.
    I modified the basic premise of the lesson to allow students to actively use Geometer's Sketchpad to discover why the distance formula works.  Students first review the usage of the Pythagorean Theorem, then gain practice in applying it to the Cartesian plane.  After students have become comfortable with working with the equation in the plane, they are prompted to make the link between the Pythagorean Theorem and the distance formula.
    When using this activity in class, students will be working in pairs to help each other recall mathematical concepts and navigate Geometer's Sketchpad.  (Students would have been given the Pythagorean Theorem and distance formula beforehand.)  Then, after the partnered section of the activity, I would have students write in their independent journals with notes taken from each page of the sketchpad file.

Lesson pages A, B, C, and D
PDF of this write-up
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