For a one-stop-shop search engine, Kayak does a solid job. Other than comparing flight prices, travelers can also search hotels, car rentals, travel packages deals, and even cruises. The ability to put in flexible dates (plus or minus up to three days of your chosen date), is a great tool to view price differences based on day of travel. Holding out for the best possible deal? Set an alert and they'll let you know when your preferred option is in your price range.