This is a thread for people to share really great applications for mobile devices. These could be apps you use on a daily basis or just occasionally, but they would be ones that really streamline the way you do things and make life easier. They can be for Android, iOS, Windows Phone, BlackBerry, Symbian or cross-platform.
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Tweetbot for iOS - Available on the iPhone, iPod, and iPad. Twitter application that is heavily gesture based in it's operation. Allows for easily setting up multiple timelines for a single user, setting up multiple accounts in the app that can be quickly switched. Provides timeline streaming when on a wifi connection so the timeline constantly updates.
Clear - To-Do List manager. Another gesture based app that streamlines setting up a list of things you need to get done. Really not a whole lot more to this than that, but it's much more user friendly than using the Reminders or Notes app that comes paired with iOS.
Weather HD - Amazing motion backgrounds, accurate realtime weather information. Quickview option if you have several cities set up to monitor weather data for at once. Much more versatile than the stock Weather app or The Weather Channel's free or paid version.
Convertbot - From the same developers as Tweetbot, this app allows you to do conversions of pretty much any basic thing you can possibly think of, including currencies which it has realtime updates for to make sure the values are correct.