An experimental Minecraft launcher which is designed for playing older versions and different editions of the game.
It currently supports all available versions of Java Edition and Xbox 360 Edition, while also having a few MinecraftEdu versions.
Java Edition also has a MultiMC-like modding options (replace minecraft.jar, add to minecraft.jar) and you can easily install modloaders with just two clicks.
I highly recommend not looking at the source code if you want to learn how a minecraft launcher works. (Code is completely shit, but hey It Just Works™)