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I've used this app many times around European jaunts, and it is a lifesaver (when my carrier chooses not to block my smartphone by accident). It's free, updated regulary and displays the transport networks of over 400 cities across the world.
You don't need a network connection once the cities are installed, but if there are transport disruptions, updates can be found by turning on your Wi-Fi.
A free but advert-supported app, this service allows you to send text messages in real-time as a group chat. In a nutshell, it is a chatroom, no matter what phone your friends use. A new feature is the ability to add voice message -- although I found the app useful based on its simplicity.
Gasbuddy is a popular app which constantly recieves great reviews. It's a free way to find cheap gas stations on-the-go in the United States and Canada.
The company update the app often, but also encourage a community through gas price reporting -- if a user reports a new bargain, then they get points towards competitions.