Using the Steam DRM is optional for developers. There are DRM free games on Steam, which means that even if Steam is down, they will still work. That said, I've

Using the Steam DRM is optional for developers. There are DRM free games on Steam, which means that even if Steam is down, they will still work. That said, I've never had problems with DRMed Steam games not working if the internet is down (you just need to make sure you check the "remember my login" thing when authenticating).