MultiMC

MultiMC is a free, open-source launcher for Minecraft. 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.

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
• Multi-platform (OS X, Linux, Windows)
• Multi-language
• Multi-themes
• Multi-Minecraft-account management

You can switch to the development version in the settings to get newer, not-yet-released features.