Everything MultiMC

MultiMC is a modded Minecraft Launcher, born from the Modded Minecraft community to make handling multiple Modded instances easier than the official Minecraft Launcher.
It was, and still is, a generally stable Launcher, and is being actively maintained.

Eventually it was forked into PolyMC, due to some disagreements with MultiMC's maintainer about the project's direction.

From then on, PolyMC continued on, and added some of their own features on top of MultiMC.
However, PolyMC eventually had some of their own internal controversy.

The lead maintainer of PolyMC, known as Lenny, banned PolyMC's contributors from the GitHub repository, and removed the PolyMC Code of Conduct. The reason for this "hostile takeover" was due to some political disagreements and controversial statements.

This led to yet another fork, known as Prism Launcher. It's generally known as another continuation of MultiMC.

All three of these projects are still actively maintained, so feel free to use any of them, but with these issues and disagreements, we recommend using Prism above Poly or Multi.