MultiMC allows you to manage multiple versions of Minecraft, Forge and the various mods involved. Sign in with your Minecraft account details and with a couple of clicks it can download various Minecraft versions and the corresponding version of Forge. I already have numerous 1.7.2 and 1.6.4 instances (Unlimited Superheroes, Fossils and Archeology, Tekkit etc.). Also this allows you to experiment with combining different mods together to check to see if they crash - you also might want to keep your differently themed mods apart. You can also organize your saves (e.g. world maps) and resource packs across the various instances (e.g. city maps and textures for use with the "cars and drives mod")