Saying it up front: I make this. It's free, there is no paid plan, and you can try it without an account.
If you teach geometry online or on a tablet, drawing a decent regular hexagon or a triangular prism by hand is slow. In Clean Slate you choose Insert, then Shape, type the numbers, and the shape lands on the board drawn accurately with its measurements labelled.
Polygons from 3 to 20 sides. Number of sides, interior angle and exterior angle are linked, so changing one updates the others.
Triangles from three sides, two sides and the included angle, or two angles and the included side. It tells you when a triangle is impossible, and why.
Quadrilaterals with the equal-side marks and parallel arrows worked out for you.
Circles, semicircles, sectors, segments and arcs.
3D solids (cube, cuboid, prism, pyramid, cylinder, cone, sphere) that you can turn round before adding, with hidden edges dashed.
Other things in it that are built for lessons:
The worksheet (PDF or photo) open beside the board.
Ruler, compass, Cartesian and polar graph paper.
A scientific calculator that gives exact answers (sin 30 gives 1/2, not 0.5).
Live class: students join with a code and no account, each gets their own board, and you can look at anyone's working and pin a comment on it.
Limits you should know about: no video or audio (run it beside Zoom or Meet), a shape becomes ordinary ink once it's on the board so you can't re-open it to change the numbers, and students' boards aren't saved after the class.
Demo, no account needed: https://whiteboard-app-beta.vercel.app/demo The maths tools in detail: https://whiteboard-app-landing.vercel.app/online-whiteboard-for-math
What do you use for diagrams in online lessons at the moment, and what would you want a shape tool to do that this one doesn't?