MultiMC - Overview: MultiMC is a free, open source launcher for Minecraft, managed by Forkk13and peterixxx...
GDLauncher - GDLauncher is a custom Minecraft launcher with simplicity and power in mind. It allows users to create custom instances (also with forge), download curse modpacks and mods and more!
PolyMC - Fork of MultiMC with additional features like mod downloading and supporting other package formats like flatpak or appimage
Crystal Launcher - Crystal Launcher is a Minecraft launcher and mod packs installer that allows you to install almost any mod without worrying about the complex Minecraft installers.
Badlion Client - Custom minecraft 1.7.10, 1.8.9 and 1.12.
FTB Launcher - Feed The Beast (FTB) is a client who does so you can play Minecraft but modded.