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QuantumLauncher

A simple, powerful, open-source Minecraft launcher with instance management, modding features, and more!
Main page of launcher, sidebar to select instances, and various options

Incredibly Lightweight

Uses minimal CPU and RAM!
Lighter than the vanilla launcher, common alternatives and even Task Manager!
System Monitor showing launcher using 39 MB RAM and 3% CPU

Easy Modding

Download and manage your mods and mod-loaders at one place!
Viewing list of installed mods, with buttons to manage them Browsing the built-in mod store, options to change backends (CurseForge/Modrinth) and content (Mods/Resource Packs/Shaders/Modpacks)

Minecraft with RLCraft modpack launched by QuantumLauncher
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Discord
Matrix
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Downloads
macOS ARM (Apple M1/M2/M...) macOS Intel (before 2020-ish)
Not sure which one you have? Click → About This Mac
NOTE: On macOS, if you have a "corrupted", "damaged" or "unverified" error, run this in the terminal: (more info) 
xattr -rc /Applications/quantum-launcher.app
Linux x86_64 (Recommended)
Linux aarch64 (Asahi/Snapdragon/Raspberry Pi) Linux armv7h 32-bit (old Raspberry Pi)
Or install it as a flatpak 
flatpak remote-add quantum-launcher oci+https://mrmayman.github.io/ql-flatpak-remote/
flatpak install io.github.Mrmayman.QuantumLauncher
Or as a Fedora COPR 
sudo dnf copr enable apicalshark/quantum-launcher-rpm
sudo dnf in quantum-launcher