Momondo
Momondo is an easy-to-use flight comparison site (it's also a convenient app for your phone) that can save you hundreds of dollars. You simply enter the dates you'd like to travel and any other specifics, and it shows you a list of flights. It also gives you the option to filter by specifics like "quickest,""best," and "cheapest." One of Momondo's best features is its price comparison calendar, which shows you the cheapest days to fly so you can adjust your trip to save money.
Expedia
As one of the more well-known travel booking sites, you're probably already familiar with Expedia. While Expedia doesn't pull from as many third party ticket distributers as sites like Momondo, meaning you might end up spending a little more, it is very trustworthy and time-tested site. It also has a "show flexible dates" option on the search page, which is a great way to see whether cheaper flights are available if you shift your trip a day or two.
Skyscanner
Skyscanner works with hundreds of other travel providers to find the best fares, so this is a great option if you're looking to keep things as cheap as possible. Of course, many of the these third party booking services will have catches, like massive cancelation fees, so make sure to do your research. If you see a great flight available from a booking sites you've never heard of, you can at least take some comfort in the fact that Skyscanner shows user star ratings for each.
Kayak
Kayak was one of the first sites of its kind, and it's still one of the best. It makes it easy to compare flight prices from different sources, and it even gives users the ability to search for flights to "Anywhere," making it our top pick for the spontaneous adventurer. Kayak also features an "Our Advice" box that lets you know whether Kayak thinks you should buy now or wait depending on how the prices are projected to fall or climb in the coming 7 days.
Kiwi
Kiwi is the best option out there if you're looking to book a trip with multiple destinations. It allows you to search multiple modes of transportation and even creates an itinerary for you. All you have to do is put in where you want your journey to begin and end, and the places you want to stop along the way and how long you want to stay there. Then you simply select "find routes," and the site will put together an itinerary that mixes and matches airlines and routes for the lowest possible price.