Here are two versions of Completing the Square - a web page from NCTM and an ActivStudio flipchart.

Completing the Square web page
http://illuminations.nctm.org/ActivityDetail.aspx?ID=132

Completing the Square flipchart

Students have a choice of two ways to use this document: students specify the steps of the process and the CAS follows these instructions ("student as spectator") OR students actually enter each line (either literally or in terms of some action upon the previous line (e.g. eq2 + (b/2a)^2) and the CAS engine verifies that this line agrees algebraically with the previous line (high student involvement). Dynamic algebra in the hands of the students!
http://compasstech.com.au/TNSINTRO/TI-NspireCD/mystuff/Dynamic_Algebra_S...

A really clever idea - sort the steps in the proof of the quadratic equation. A ball moves vertically up (good!) or down (bad!) to let you know how you are progressing. I never thought that proving the QE could be this much fun!
http://nrich.maths.org/public/viewer.php?obj_id=1394