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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.
Name: Couch surfing
If you're really watching the pennies, Couch surfing is an online community of travelers who offer their couch when they're back home for a time, for free.
In the same way as a hostel or hotel, you can 'book' (request) a certain time or number of nights to 'surf' in someone's living room.
Each profile comes with ratings, peer reviews and comments on the stay, as well as location and surfer preferences.
I've used the service a few times when backpacking, and as long as you do your research, it's an easy way to find somewhere to stay for the night.