What it is
projectM is the open source engine that keeps MilkDrop presets running everywhere the original never went. Version 4 rebuilt the renderer to be mostly compatible with MilkDrop 2 presets, and the library ships inside media players and standalone apps across macOS, Windows, Linux, and Android.
For a Mac user, this is the MilkDrop experience: point it at system audio or the microphone and step through the same community preset archive.
What it costs
Free and open source. Desktop builds are on GitHub; app-store wrappers of it sometimes charge a few dollars for the packaging.
Best for
MilkDrop on anything that is not a Windows box. Developers embedding a proven visualizer engine in their own player, since libprojectM is built for exactly that.
Skip it if
You want the newest MilkDrop 3 features (Windows users should run MilkDrop 3 itself) or performance-grade control over which preset shows when, which is NestDrop’s job.