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.
You can switch to the development version in the settings to get newer, not-yet-released features.
The use of variables in custom commands is now better documented. The label shows that they need to be prefixed by $.