After looking up what a math manipulative actually was I came across the geoboard(http://en.wikipedia.org/wiki/Geoboard). It is a pretty cool contraption for teaching basic concepts of plane geometry. Read more about it through the link.

I have also been wanting to learn OpenScad for a while and this seemed like the best time to pick it up.
You can customize this but I have also included the STL for a 25 peg geoboard. Everything I could think of is all customizable.

I hope you have as much fun printing and using this as I did making it.

Update: I have reworked the board so that the number of pegs is completely customizable along the X and Y axis independantly and infinitely. The percentage of the pegs dedicated to the step and the step width are also customizable now.
I made everything that I could think of customizable. If there is a parameter that I missed let me know and I will try to add it.
I have also modified the STL by making the pegs 3mm instead of 2mm. 2 just seemed a little to thin.