SilkDrop compiles libprojectM(https://github.com/projectM-visualizer/projectm) — the open-source MilkDrop engine — to WebAssembly and renders it through WebGL 2. Point it at a browser tab or your microphone and it reacts to whatever is playing.
There is no backend. The site hands you ~10 MB on the first visit and is never involved again: rendering, preset compilation, audio analysis, search across all 9,795 presets, and every byte of storage are local. Load it once and it keeps working with the network off. There is plenty of other features as well like live presets editor, built in over 9k presets with search and favorites.
Made it for fun as a pet project.