MONEY rated the most popular travel apps, then tested each to find which ones could save you the most money. The winners:
Using data from every major airline (including Southwest), this app selects itineraries based on shortest layover, optimal departure times, and best price. Especially on overseas flights, Skyscanner often finds flights that are cheaper (by $100 or more) than you could on your own.
Crowdsourced data on fuel prices nationwide. We found an average differential of 41¢ a gallon across five metropolitan areas. Use Gasbuddy before you return the rental car.
Make a lunch or dinner reservation and earn points that can be converted to gift cards. Some restaurants offer 1,000 points, worth $10 (though the majority give you 100 points, or $1). OpenTable works at more than 33,000 restaurants worldwide.
Booking.com links to more than 800,000 hotels with especially good deals (up to 50% off the list price) on same-day reservations.
Search for a route, and the Hopper tool shows you a daily breakdown of the lowest airfares. If your departure dates are flexible, you can save hundreds of dollars. The app will also alert you when the price drops.