Live Geometry is a free and open source interactive geometry website. It has a colorful gallery of geometry drawings illustrating various geometry concepts. It allows you to create interactive ruler and compass constructions, work with coordinates and experiment by dragging points and figures.
The source code is at github.com/KirillOsenkov/LiveGeometry. It is written in .NET and Avalonia.
Also here: Catriona Agg's geometry puzzles made interactive; the same gallery in plain JavaScript; and how to embed a live drawing in your own page.