MultiMC is a free, open source launcher for Minecraft, managed by Forkk13, peterixxx and drayshak. It allows you to have multiple, separate instances of Minecraft (each with their own mods, texture packs, saves, etc) and helps you manage them and their associated options with a simple interface.

Official site: multimc.org

Features

Manage multiple instances of Minecraft at once
Start Minecraft with a custom resolution
Change Java's runtime options (including memory options)
Shows Minecraft's console output in a colour coded window
Kill Minecraft easily if it crashes / freezes
Custom icons and groups for instances
Forge integration (automatic installation, version downloads, mod management)
LWJGL version changing built-in
Supports every version of Minecraft that the vanilla launcher does



To install:
Extract the archive to a new folder and run it. Remember to make it executable before trying to