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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.
Available as a free or pro version, this app gives you access to a search function and booking for flights, cars and hotels if there is a change of plan -- which can be expensive and heart-breaking if you're minding your budget.
The Kayak app is one of the more popular search functions available. It is possible to compare and book flights, check the status of your flight, manage a trip itinerary and find information including baggage fees and airline numbers.