Dynamic Tweets offers a tweet deferral service like a few others we have reviewed. It works well scheduling out tweets for reminders and marketing as the website shows. Also, if you have something important to say in the middle of the night, when not many would see it in the great stream that is twitter, having it set to publish during normal hours just makes sense.
You do have to register for the service, but I wish they had implemented oAuth already. This means you will create an account with them and then give your Twitter credentials over to them as well. I always think the worst and I am not saying they would use it harmfully.
Now they do most of the end user service for free, with a fremium model for professional and corporate users. The full chart can be found right here. Such things as sending predefined tweets based on keywords is saved for money paying customers. Multiple Twitter accounts are available to any level though.