Some apps can really change for good the user's experience, adding or replacing features. With this thread, I want a place where users can share the apps in their opinion are a "must-have", and maybe finding out new great apps they might not know about.   Please, do not post social media or game apps, because those are either known or entertaining apps, the idea is instead to share productivity and utility apps.   So, here's my picks:   OS: Android   SwiftKey Keyboard (free): This very u