When your phone is in your pocket, full volume notifications are necessary if you don't want to miss a message. But when you already have your phone out and unlocked, there's no need for them to be loud and annoying, especially if you're already using the app they originate from (e.g. message apps).
Awake/Sleep Volume Adjuster lets you set custom notification volume levels for when your phone is asleep and when it's awake.
This is my first app, so I apologize for any issues it may have, but I hope people will find it useful. Either way, it's been nice to have just for myself!