Sketchpad® Dynamic Geometry® software gives students a tangible, visual way to learn mathematics that increases their engagement, understanding, and achievement. And it's not just for geometry. Use it for elementary and middle school math, algebra, precalculus, and calculus.
KSEG (written whilst the author was still in high school) is a free alternative which is open source (you can see how he made it) most of the software listed is **function plotters**, _not_ geometry. (“dynamic” geometry just means that the circles can be resized, the lines moved around, and things will stay connected as you defined them) In general, dynamic-geometry software would improve mathematics classes more than almost anything else, because it allows you to _play_ and perhaps even notice some little “theorems” of your own. If you like these, check out surfer (https://imaginary.org/program/surfer) — a 3D equation plotter which shows how simple algebra (polynomials of any power simultaneously multiplying or adding x, y, and z) looks in visual format (the "variety" i.e. set of points where that polynomial vanishes).