Easilydo doesn't necessarily replace an existing app, but it's kind of like a smart proactive to-do list for the iPhone. The app uses similar techniques to others - slurping in your email and Facebook and everything else and applying natural language processing and other logic - and suggests simple tasks it can complete for you.
Twitter users can see which of their Vine posts drive the most engagement and impressions and which users are the most influential in making the posts go viral.