The following list contains active links to Geometer's Sketchpad (GSP) scripts. When opened in GSP, one may create the particular construction by simply starting with the given criteria named in that portion of the script. I chose these particular twenty because they are both useful and interesting.
7a. Orthocenter, Mid-segment triangle
10. Center of Nine point circle
12. Triangle Centers (H, G, C, and I)
14. Equilateral triangle, given a side
16. Isosceles triangle, given base and altitude
17. Regular Pentagon, given a radius.
18. Regular Pentagon, given a side.
19. Regular Hexagon, given a side
20. Regular Octagon, given a side